Highlights in a man's hair that is wholly unattractive: generally an indicator of one's toolish nature. Look for a pompous air and cheesy smile. Guylights are perhaps a descendant of the late 90s tendency to bleach one's hair until it looked like a skunk had pissed on it.
Barbara: Suzy, what do you think of him over there?
Suzy: With the technicolor hair? I could never date a man with guylights, Barb.
a man (not necessarily gay, but in most cases, yes) who dyes his hair blonde
Guy1: Did you see that guy over there?
Guy2: Yeah, he had GuyLights. So gay. (Assumption)