From Mercury silver suits to orange Space Shuttle ones, color of space suits have changed over years. Is there any reason for it?
You are referring to the materials protecting from insulation and the outer layers of space suits. Look carefully, they have not seen much change since the Apollo era.
In Mercury, the suits were still at an rather early stage of development. They were intended as pressure-suits protecting the astronauts from decompression in case there was a leak in the capsule. They were not intended for EVA. In a similar way, the orange suits used in space shuttles at the end of the program were just pressure-suits.
Suits for EVAs, on the contrary, have been white to grey for more than 40 years now, both in the US and in Russia.
EDIT: http://en.wikipedia.org/wiki/Advanced_Crew_Escape_Suit "The orange color allows rescue units to easily spot the astronauts in the case of an Orbiter bailout over the ocean."
EDIT2: This explains why EVA suits are usually white: http://spaceflight.nasa.gov/station/eva/white.html