How does the color of the dorsal and ventral sides of a fish provide protection?

The dorsal blend in with the water, and the ventral blends in with the bottom surface. So they can tell if its dangerous if it is brightly colored. ( bright red frog= poisonous and non-tasty frog) :D - < I'm not sure if this right...I just guess..>

P.S.

This is exactly the same question on my science worksheet!