I totally agree with PP. I have done a 5k Colour Run in my local park organised by The Irish Cancer Society for the last few years. This is the most relaxed of the runs but even still very few people are in costumes, I dress up every year and its usually just a sea of white charity tshirts and black leggings. Even in this run which is very much a family event, there are some serious runners, who just literally race the course, speeding by everyone. You dont find many people randomly talking, just the groups of friends or families.
Afterwards dont worry about just changing at your car. I dont drive and every year bring a change of clothes which I leave in the bag drop area. After the event I just go to the side and completely change, even cleaning down with wet flannels and no one takes one bit of notice.