I drew this a few weeks ago, when I didn't have a lot of time to dedicate to art. Everything is very simple, so I figured it would make an easy-to-follow step-by-step piece... This was done in Photoshop CS3. Everything is in its own layer, which might seem like a lot of work, but allows you to be lazy later on...
Rough sketch. This was done in its own layer on top of a white background layer. I used the hard-edged round paintbrush tool in gray at about 70% opacity.
Lines. I set the hard-edged round brush to have shape dynamics. I made a new layer on top of the rough sketch and traced with the regular round brush in black at 100% opacity. When I finished, I made the sketch layer invisible.
Rough colors. Basically, I threw a bunch of colors all over the place, like a three-year-old. It's messy, but everything is in it's own layer (background, mouse's feet and tail, mouse's body), so I can erase the mess later.
Final. (Hey, I told you it was a speedily-done pic...) I blended the colors by using the eyedropper tool and painting at an opacity of about %4o on top of other colors, and then using the eyedropper tool to use the newly blended colors. I fiddled around with mixing the colors until I was happy with them. I also used the soft round brush at a low opacity (about 15%) in pink to add blush to the mouse's ears and face. Now the "everything has its own layer" bit comes in. Since each colored section is separate from the others, you can just go in and erase anything outside of the lines in each layer.