Well, colors can be both pretty and informative.

Had to take Color Theory to get my BFA. We learned things like the "speed" of colors: yellow is the "fastest" and purple is the "slowest". Only a little yellow is needed to balance out a lot of purple. Same for balancing the other colors. You use these balances for guiding the eye (lots of yellow on a purple field).

When you keep this in mind, diagrams or not, then the output will be "pretty" (not jarring) and "informative" (guiding the eye to the most relevant info).