Tip of the day is;
Experiment with brush blend modes!
If you look at the example above, the left image is how people usually have brushes set. Blend mode set at normal, with the opacity turned down. On the right, we have the blend mode set at multiply. This means every stroke put down will be like a new multiply layer has been applied. I like doing things this way because it keeps everything vibrant, and the colors change in hue and value instead of the hue staying constant.
There are other blend modes you can experiment with, and I encourage you to give it a try!