I’m filling out a form for one of those engraved bricks at the new Colts stadium for my girlfriend, and I want to make sure it’s correct. There’s no fixing it!
In this instance, “Colts” is attributive. It’s not genitive. So “Colts Fan” is correct.
The genitive case is a way of indicating a relationship between two nouns. “Season’s greetings” doesn’t mean the greetings belong to the season, but rather that the greetings are OF the season. A week’s wages = the wages of a week. A Midsummer Night’s Dream = The Dream of a Midsummer Night.
I don’t think there’s anything wrong with that. In this instance, Baltimore is short for the Baltimore Colts. If she were a Utah Jazz fan, you can say she is the Jazz’s #1 fan or Utah’s #1 fan.