"The book is colourful" is not acceptable?
I believe it's acceptable. Just report to DL.
But in English coloured and colourful mean different things - coloured just means it is in colours, colourful in pictures would mean bright/striking colours, and in words is a euphemism for swear words ('he used colourful language').
We'd usually use 'coloured' as in: 'The pictures in the colouring book are coloured in' or 'the book is coloured yellow, red and blue'.