I'd add that if you want to compliment someone - especially someone who you could theoetically have a romantic relationship with - try very hard to make it about something they did, and not something they are.

Telling a guy his hair color is nice or telling a girl she has pretty eyes may both be true, but assuming no cosmetics, they didn't do anything to achieve them. Telling a person that you admire how discplined they train in the gym is a compliment on something they actively did and you recognized.