Great idea! Touching up paint marks or chips makes a real difference. In fact, I've just dome the same myself. I found a great tip for how to store felt over paint written by Danny Seo in the first issue of "Naturally".
"If you only need a little bit of left over paint to save for touch ups down the road try this upcycling idea: Use old plastic water bottles. Start with a clean, dry water bottle and lace a funnel on the top. Pour leftover paint into the bottle until it's about 95% full, remove the funnel and toss in about 6 glass marbles. Replace the cap tightly and secure it with a few strips of painters tape. When it's time to use the paint, shake it; the marbles will blend the paint, and you'll be good to go to till in any nicks or scratches."