Adding white paint will make something lighter, but it will also make it duller, especially in combination with black paint.
Our mural is very dull compared to the colors of paint that come out of the bottles. Almost all of the colors need to be mixed with both black and white to reduce the saturation—to make it "greyer."
I also noticed expressions of confusion when I would look at a student's palette (consisting entirely of shades of blue paint) and tell them there was too much green. Though they had only used "blue" colors, those blues tended toward the green end of blue, rather than the purple end of blue.
I created these reference sheets to illustrate the differences (click to enlarge):