One of the main difficulties of using Linux is finding the right software. Often, FOSS offers multiple choices of software for any one task, the trick is finding the right one.
For painting, Krita has been an old and faithful standby for many Linux users. As an amateur and unskilled artist, I cannot say much about it other than that I have found it sufficient.
However, there is another option: MyPaint, which is free software and also available on Windows. From my brief experiment with it, I have found its user interface to be more accessible than Krita's, its default set of brushes more than satisfy my personal art hobbies, and it works with my graphics tablet out of the box.
Your results may vary, but it's definitely worth checking out along with Krita if you are painting on Linux.