Lightening your acrylic paint colors with white paint is called tinting. Each color has a limit on just how much lighter you can make it. This is called tinting strength. Dark colors have a higher tinting strength because they’re darker to start with and so have more room to be made lighter.

Here are the colors from the color wheel going from pure color on the left side to pure white on the right. A gradation between the two shows how many colors exist between.

You may want to tint a color in your painting for a highlight if the sun is touching it. Just add a bit of white paint to your color. Add it gradually to create a strip of paint choices that range from pure white to pure color.