You use animated track mattes. The trick is creating the track matte. You can use dozens of techniques but most if not all involve a bunch of hand work.
Here's what a Google search turned up as the first offering when searching "After Effects reveal painting tutorial." I didn't watch the tutorial but it should get you started. As I said in the first sentence, there are hundreds of different ways you can do this technique but all involve animated track mattes.
After you begin to understand animated masks, as Rick says, you will want to explore the far more complex watercolor effects that will simulate sumi e, brush and pen, and other wash techniques. Depending on your need for realism and your desire for a unique and elegant style, you can plan on investing about four hours for every on-screen second of animation. Well worth the effort rand time and you will end up knowing much about After Effects than you do now.
Thanks i'm gonna try that out!
I must admit i didn't even what to search for on google.
The Help section of the AE application will take you to many dozens of offline resources. Once you hit a few of them you will start to get familiar with the search terms. But you can start with youtube and just enter AFTER EFFECTS TUTORIALS and then try to narrow down what you're trying to find like painting effects or water color transitions. Warning, lots of these things on youtube are dangerously inept.