About shrimps

Hi everybody!
I've a fresh water tank with some fish and 4 shrimps.
First question:
Three of those are transparent but one, the smaller, changes its color. It
goes from brown to orange to blue. It looks like a PH tester (i mean for the
changes in color). Is that related with the shrimp's gender or what? Why
others are always transparent?
Second question:
Two of those shrimps are as big as other fishes. Could they attack them (for
eating purpouse)?
Third and last question:
Is it normal for shrimps to come in surface to eat (like other fishes)?
Thanks a lot
Max from Rome

PS Sorry for my really bad english. Please correct error if you like, it
could help me improving my english.