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The theory behind colouring books for adults is that they relieve the stress and anger of our daily lives, even if their very existence makes some critics purple-faced with rage.
These books are a bona fide publishing industry trend, though: boosting business for retailer WHSmith in the UK, fuelling a rise in non-fiction sales for US publishers, and helping Amazon chill out some of its customers in south London.
This being 2016, inevitably there are now apps for that. iOS app Recolor is the work of developer Sumoing, whose Repix, Relook and Camu photography apps have been downloaded more than 25m times.
The Finnish company has created more than 200 illustrations for the app, with a simple set of colouring tools to change individual areas of each picture with a tap.
This being 2016 (still) the app uses the freemium model that’s now dominant on the major app stores: it can be downloaded and used for free, but individual illustration packs – from animals, fishes and insects to isometric images and bejewelled skulls – can be bought using in-app purchases.