So this next site is not a painting tool BUT it is a valuable tool for an artist. The Color Scheme Designer is a great tool if you are looking for colour combinations.
At the top left you have the different kinds of colour harmonies that you can choose from - I have chosen Complementary on the example above. At the bottom you can select different hues and values/saturation (Adjust Scheme). You can then see the list of colours that it produces (Color List). You can enter your own value by changing the RGB value just above the Color List.
So I took the caramel colour that I used with the flowers in my previous blog and was interested to see that I had pretty much used it's complementary colour without actually realising (the blue):
Most of the people who use this site, do so to pick colour schemes for web sites and if you select Light Page Example or Dark Page Example which is an option at the bottom right, you can see what your web site might look like. However, I think it's a great tool if you are doing collages to get good colour combinations.