It could be any trait, it's just to clean up the genes so that the ones that aren't disireable don't show up in later generations. Also when we use the term cull, we sometimes don't mean to kill them off, we just remove them from our breeding population.
That's what I do at the moment, and just sell the less colourful shrimp as lower grade.
Growing up in a semi rural area I always assumed culling to be killing, it's good to know they are not being popped off for not looking perfect. I know I will probably be seen as a bit soft for this but I see it as a bit harsh to kill mine off when I can just as easily re-home them.