The font is white, the background is black, the links are blue - but now I'm noticing that sidebar is gray. Does the gray sidebar count as a fourth color? 

You can have 5 colours.

I think this used as new color. Recolor the sidebar!

I was doing Puzzle Jam and Mini Game Jam at the same time (with the same deadline). And I realized 30 minutes ago that with a few tweaks, my Puzzle Jam game (https://charcharchara.itch.io/monochromatic-escape-room) would have been PERFECT for the five color limitation. Ah well. :D