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. "

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