You can digitally paint not just in Corel Painter but in Adobe Photoshop as well. Some people do amazing work painting in Photoshop. Here is a very basic video on the mechanics of actually blending from color A to color B using a Wacom tablet.
This is not necessarily the best way to work, and I am far from the best, most skilled painter, but this is how I work. Your feedback and improvements are welcome.
Topics in this clip include smudging, Gaussian blur, Wacom pen-pressure opacity, and color blending, but not color theory or color mixing, or speedpainting / art instruction -- that's beyond the scope of this.
I think that annoying skipping in the video is from the interaction between CS4 and my screencasting software. I need to investigate further.