Drawing Box is a free drawing app with several tools and a million colors to choose from - maybe not a million colors, but close. There is also a paid version of this app for $2.99. I think it might be worth the investment. We'll talk about the paid tools momentarily.
In the free version you have access to various paper types and backgrounds as well as pencils, brushes, rollers, stamps, a symmetry tool and paint colors. You can create up to 6 drawings but each drawing can be saved the camera roll so once you've made your masterpiece you can save it and then delete it to make room for more.
The paid version provides more features such as color mixing, tool pressure sensitivity, importing pictures and the ability to export your drawing as a movie. This last feature looks really cool - it shows from start to finish how a drawing was developed.
Overall, I give this app to thumbs up - even the free version is great. There are also Drawing Box videos available on YouTube you might want to check out. They show an older version of the app, but it will give you the general idea.