The trick I've found with Overlay layers is to paint with mid-tones. (around 50% on the value scale). You can use any colors, as long as you stick to a mid-value, and it will simply modify the hue/saturation. When you use lighter or darker values that will push and pull the values of your image. That can be useful. I often use Overlay mode to both alter color and values. You just have to be mindful of this. If you only want to modify color, then choose mid-values from the color window.