Pixia is a powerful paint and drawing program.
There are a host of selection and paint tools, colour tweaks, layer support, masks, effects and more.
Click on "Colour Adjustment", for instance, and you don't just get some RGB sliders. Instead there's a full curve correction tool, with a choice of curve type (Bezier/ Spline/ Straight), number of points, and the value you're tweaking (Red, Green, Blue, RGB, Cyan, Magenta, Yellow, Hue, Saturation, more).
What you don't get, unfortunately, is any real documentation on how to use all this power. Apart from a brief README.txt which doesn't seem to have been updated for 3 years and two major version upgrades.
Still, the basics work as you'd expect - choose a colour, texture, brush size, click and drag on the canvas to draw - so as long as you're willing to experiment, there's plenty of fun to be had here.