Adult colouring books? Inevitably, there's an app for that

Developer behind Repix and Relook photography apps turns its attention to stressed-out smartphone and tablet users

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.