Millie Marotta tells us about drawing her colouring book iPad app as it hits 250,000 downloads

Amazon's best-selling author of 2015 tells us about turning her colouring books for grownups into an iPad app to use with your fingers or stylus (including Apple's Pencil).

After Millie Marotta topped Amazon's 2015 bestseller list with her adult colouring books, the artist has found similar success with a freemium app Millie Marotta’s Colouring Adventures – which has been downloaded over 250,000 times.

The milestone coincides with an update to the app that adds two 79p packs of drawings to colour in: Birds and Wild Savannah (the free version of the app includes only six drawings to colour). There's also a new Colour Picker, a brush to colour with and a My Album section to keep finished artwork in.

This growing number of drawings is designed to be one of the main ways it appeals over the printed books. Millie tells us that another possibility offered by the app is the ability to colour-in images multiple times in differing ways.

"They can experiment with different palettes, recolour, edit and erase to their heart's content. It’s a very different experience from the tactile nature of using pens and pencils in the books but is a very unique, satisfying and dynamic experience that is equally as inspiring. "

As one of the first apps compatible with the Apple Pencil, it is clearly aimed at stylus users. But those without (or those who crave a challenge) can use their fingers to colour the intricate drawings. After all, mistakes are easily erasable and, thanks to the zoom function, aren’t made often anyway.

We asked Millie if she had to draw or approach your artworks in any way differently to make them suitable for colouring-in the app rather than in a book.

"Not really," she replies, "and I think that's actually one of the great things about this app, that it is able to provide people with illustrations that retain the charm and delicate nature of hand-drawn images from within a digital platform. The only thing I had to do differently was to scan the illustrations at a higher resolution, to allow for the user to be able to zoom in and out as they colour."

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The app was created with app developer Touchpress for Pavilion Books.