As someone who needs to use every color in the pack of highlighters to do any assignment, believe me when I say that this is the definitive ranking of all the colors from worst to best.
6. Purple
It's too dark to see what you're highlighting, so if this is your top choice, I don't trust you.
5. Blue
Sometimes blue is nice, but sometimes it's way too dark. A dying blue highlighter is an acceptable choice, but a brand new one? No.
4. Pink
The pink is always way too bright and hurts my eyes.
3. Green
Green is a nice middle-ground where it's not too dark, not too light; not too bright, not too dull.
2. Yellow
Yellow is the classic for a reason, but it's a little too basic sometimes.
1. Orange
I will die on this hill. Orange is the perfect highlighter, and I will tolerate no slander. It is the perfect brightness and the perfect shade. Don't even try to change my mind.