We dont yet have many details on this, but what a cool idea, and very helpful for the entire hobby community.
via the Warhammer Community
https://www.warhammer-community.com/2017/08/21/coming-soon-the-citadel-paint-system-in-your-pocket/
The Citadel Paint System is an easy, straightforward method for painting awesome looking models, and this September it’s going to get even easier. There’s a new app on the way designed to help you paint like a pro, whether you’re just starting out or you’re a veteran looking for a handy pocket reference guide.