Why is it important to fill the page with colors when highlighting?

When you highlight something, you make it special. You point it out. When you keep adding colors, you cease making things special but start making them confusing. When you come back to the material a a later time, you say, "Why is this red? Why is that blue?" You create more questions than you make things important. The main points of your outline are the most important. The sub points are second most important. The rest is detail. You highlight to clarify, not to confuse.

Highlighting only helps important things stand out if there isn't too much of it.