Heh... believe me, a good program isn't a replacement for skill. :-)
I've never heard of Xara X or PhotoImpact, but Flash isn't exactly a drawing program... more of animation and/or presentations. So, we'll leave that alone for the time being.
Everybody, their brother, twelve of their best friends, and their dog all seem to say Photoshop is the main program to get for drawing. I dunno, the thing is WAY too expensive for rational comprehension...
Past that would be Paint Shop Pro. It's by FAR cheaper, and it does nearly everything Photoshop can. It can also use Photoshop plugins. This would be my program of choice on Windows, though I don't use it much for coloring purposes.
If you're going for free, you can't go wrong with <a href="http://www.gimp.org/">GIMP</a>. Completely free, has a Linux version, and is mighty spiffy. A trick to use if you're used to Photoshop or Paint Shop Pro, though. (<a href="http://www.gimp.org/win32">Windows version</a>) This is my main coloring bit of choice.
But, those are my suggestions.