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.