In fighter collection screen why some purple, gold (urf mobile)

I don't have a great answer but it gives you some idea about the quality of the fighter. The order is gold, purple, yellow, grey. The better classes have a higher baseline (and probably maximum) power and usually have more abilities to choose from. I would spend your cards on mostly gold and purple fighters and just use your grey fighters to get coins. The exclamation point means you have a new fighter that you've just received in that weight class. It's just letting you know where to find them.