1. Colour doesn't have to be your main focus – you can think of texture too.
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When it comes to colouring, think about all aspects of the page. "Some people like to colour in every single tiny little detail on a page", Millie Marotta, author of Animal Kingdom and Tropical Wonderland, tells BuzzFeed Life.
"And then you've got other people who like to colour in much larger areas, and will colour in an entire section that has details drawn. But they'll allow the line work to show through the colour – there'll be colour but there'll be texture underneath."
2. Work with the detail on the page.
Catarina Gomes.
When it comes to illustration, Marotta doesn't have colour schemes in her head: "I don't really have a colour scheme in mind myself when working on an illustration, but that I do think about all the wonderful ways in which other people might colour the image in their own unique style.
"I think about the size of shapes that people are going to be colouring in. I think about the details, that it's an interesting composition that people can work with – that it's got plenty of variety, very intricate areas, and larger more empty areas to colour."
3. You should add to the drawings on the page.
"I also quite like to encourage people to add to the actual drawing with their own drawing, rather than just colouring. On some pages I've recreated a much more empty, less detailed version of the drawing on the opposite page. So people are encouraged to either colour or fill in their own patterns and textures."
4. Pencils are your best option.
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"Personally, I prefer pencils. I'll always use pencils over any other material for colouring, just because I love the fact that they're so versatile. I like that they are fine enough that you can work in lots of detail.
"I love that you can blend colours into one another and get a new shade. Just from one green pencil, you can get 20 shades of that one colour, which you can't get so much with coloured markers or gel pens. For their versatility, pencils have always been my favourite, hands down."
5. Markers tend to be more trouble than they're worth.
Kimberley Heales
"In terms of markers, each to their own. I think with markers you have to be careful what you choose. If it's something with a really heavy ink flow you might have a problem. The ink might bleed through the page, or the inks might run into one another or on the page."
6. Although it depends how heavy your hand is.
"It also really depends on people's colouring technique. Some people have a really light touch, so there's no problem when they're using pens. Other people have a slightly heavier hand and they might find that their inks are bleeding through the page, or causing a shadow through the page.
"But I don't have anything against markers or pens at all. I've seen some amazing results from people who used pens with my books."
7. Choose an illustrator whose interests are the same as yours.
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"I tend to gravitate towards drawing birds," says Marotta.
"But I don't exclusively draw birds. I studied wildlife illustration at college in Wales. We would go to Skomer Island off the coast of Pembrokeshire to draw the wildlife there.
"There's a seabird breeding ground there. What we had at our fingertips in terms of being able to go out and draw from life obviously fed into what I was drawing at the time. I include wildlife in my work that lives in my back garden, or is at my doorstep."
8. And choose an illustrator whose style you admire too.
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Marotta's style syncs well with colouring – lots of details and open spaces. "I met with my publisher and took some samples along of the kind of illustrations that I wanted to include in the book," she says.
"I took along some black and white examples of existing work that was in the style of images I thought people would like to colour in. I think that's why it works so well – I was already working in a style that fitted this type of book."
9. Because colouring books are illustrated with the user in mind.
It's not a case of adapting existing illustrations for colouring, says Marotta. "When I'm illustrating for colouring books, I try to make sure there's plenty of variety in terms of content. I don't want people to get bored as they race through the book."
10. You can request your favourite animal to colour.
"I get some really really random requests for animals sometimes," Marotta says. "And people will often colour an image and send it to me, with quite a personal message – they chose to colour in an elephant for such and such a reason. It's nice to have a personal touch."
11. There's no set amount you should spend on your materials.
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"I think that it really depends on the brand. I've done quite a bit of research in testing out different pens and pencils. I've probably got about 15 different brands of pencils here in the studio. I've got one set that I spent about £4 on, that colour brilliantly. Then I've got another set of say 10 or 12 that cost £14.99 but they also colour beautifully.
"I think the thing to look out for is to try and choose a pencil that isn't too hard. Often if the colouring pencils are too hard then the pigment doesn't come out on the paper. You don't get that sort of vibrancy in the colours.
"Also if they're too hard, you'll find it difficult to blend the colours together and to shade. I don't think it's a case of spending loads of money, I think it's a case of finding the right brand."
12. Don't be afraid to mix your materials.
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Mixing materials has an incredible effect, Marotta says. "People have been using watercolours. Very very lightly – not too watery – but they're using a thin wash of watercolour or pastels to put background colour on the image. Then they'll fill in the details with pens or pencils on top.
"You don't have to stick with just one material. You can create a piece that is completed by using a mixture of materials."
13. If you're feeling overwhelmed, it helps to limit your colour scheme.
"What I've noticed is that lots of people are making a choice about choosing a particular palette of colours, and only working with those in an image. They might give themselves a theme.
"For example only use autumnal colours, or really summery colours. I think that really narrows down the options for people a bit. Other people just choose calming, harmonious colours, or they might only choose warm colours or only cool colours."
14. And then you can really limit it.
Alexandra Wegner
Emma Chapman
"I think it can make for a really striking image, using a completely monochrome palette, and only colouring an entire image with one colour. This won't work as well with pens, for example, but I've seen some amazing examples of images that have been coloured just with 20 or 30 different shades of blue.
"It doesn't always mean using just one pencil: You can use three or four different pencils and it can make for a really striking, interesting image."
15. Reach out to other fans for inspiration.
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"In terms of inspiration, I'd direct people towards the huge sharing over social media. People aren't just sharing the pictures, they're also sharing colouring tips or techniques, or sharing ideas with people about their favourite brands of pencils or pens. These are the people who are actually using the books, so what better source of advice than the other users?"
(Why not try the Animal Kingdom gallery.)
16. Take things step by step.
"That's the thing about colouring: You shouldn't over think it or worry about if you're doing the right thing or the wrong thing. It's just an activity that should bring you joy. You shouldn't worry about it. It's supposed to take some of your stress away, not cause you more stress.
"If you think, 'Oh I've got a huge blank area here that I need to fill, and I've only got this skinny little pencil', think about using a material that might cover that large area better."
17. And enjoy it!
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"I have found that as I worked from my first book, through to Wild Savannah, which will be my third, while I'm drawing the images, I think more about how they're going to be coloured in.
"I think about what materials they might be using, what colours they might be using on that page. I think at the beginning that wasn't something that was in my mind so much. But now that I see the things people are doing, it almost feels like there's more pressure to create something really beautiful. I feel like I almost have to give them something amazing to work with – as they've responded with something so beautiful for the first two books."
18. Because authors enjoy seeing the results as much as you do.
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"I think that at the beginning I did feel very protective over those images. I felt a bit nervous about what people were going to do with them and how people were going to work with them," Marotta says.
"But actually I found very quickly, as soon as people started to share their images over social media, and I could see the fantastic things people were doing, all these worries washed away. I found it really fulfilling to see how much pleasure people were getting out of working with the illustrations. I could see that they were turning them into something that wasn't mine any more. It was something unique and individual for each person. I've enjoyed that aspect of it; it feels like a collaboration with each person."
Animal Kingdom Deluxe Edition by Millie Marotta is out now (Pavilion Books) and is available from Amazon.
CORRECTION
The island off the coast of Pembrokeshire that Millie Marotta visited is called Skomer Island. An earlier version of this post got the name wrong due to a transcription error.