To modulate is to make a gradual change within whatever is being modulated. In music, a single chord can modulate to another key simply by passing through a chord that both keys share. In painting, one way we can modulate color is to use this same principle--move from one color to another THROUGH a hue each has in common.
Within this image of an autumn tree, we can see greens reflecting onto the orange leaves.
One way to modulate this in paint is to use the hue both green and orange have in common: yellow. We'll take our color from the small sample I have circled.
In the example above, I've modulated within a high saturation. If we change the saturation, the modulation looks more realistic for the shadowed areas.
What a fun idea to play with on a Saturday afternoon!