Good color mixing tools for painting include TryColors.com and HousePaintingTutorials.com. TryColors.com provides primary and secondary colors at the bottom. Click on one of the colors to add a drop of that color to the palette. Continue adding drops of each color to see what the finished result is.
House Painting Tutorials provides even more assistance for mixing colors. It explains the difference between primary and secondary colors and how mixing works. It also gives insight into other colors, including intermediary and tertiary colors. The website shows different types of palettes and paint color wheels to provide an idea of how color mixing works for different painting projects.