WHY are some beetles that really pretty oil-slick color?

Question

I know almost all animals are colored to protect them from predators. I have seen some beetles that are black, but their shell shines blue, green, and purple on the black, and it's weird that it is so pretty. I cannot imagine why that coloring would evolve, and how it benefits their survival. I apprecaite any help, especially if it's not too graphic, because bugs actually freak me out.
Thanks!

Both of your answers are too awesome. I will have to put it to a vote :)

Answer 1

It might be this reason, given for one particularly impressive type of beetle:
"When lifting their wings to fly, this bug reveals bright flashes of iridescent green and yellow, momentarily stunning predators and allowing escape. "

http://www.buginabox.com/calcas.html

Answer 2

This actually has an interesting cause. Such iridescent colors found in nature is usually due to light intereference or diffraction. A thin oil slick would produce light intereference, but it's tiny, regular-spaced structures like microscopie venetian blinds that causes light diffraction and produces iridescent colors. Insects are almost like nanomachines, using mostly hard chitin for body, leg, and wing structures. Viewed microscopically, the chitin structures are frequently complex, repetitious structures, examples of optimal engineering, really. As a byproduct, it diffracts light and produces the iridescent colors.