Every fantroll has a theme. For example:
Halloween, scientist, teacher, librarian, mythical creatures, and of course gods/goddesses.
Keep the theme pretty simple and kaboom it's an easy step and also important one when making a fantroll.

Also take in account my fantroll Sehine. He is a quite old one of mine I made about 2 or 1 year(s) ago (picture reference at top)I shall thank roxy for drawing that. I didn't give her, specific detail on his shirt but his shirt was red with his a (green) snowman symbol. This fantroll had a respective theme to him. He was based off of Christmas, his lusus was an abominal snowman, his eyes were two colors (like sollux's which is perfectly fine because it's canon that trolls can have different eye colors when old enough) and he also has a perfect cute red and green striped scarf, which also goes with his theme. This is a perfect example of sticking with the theme, his name was Sehine Hermine, though his name wasn't perfect it still fit the bill.