The colored 600's first showed up as part of the Build Your Own program Gerber offered back in 2001. A person could fill out an online form and select which specific blades, tools, and plier head they wanted on their new 600, as well as which finish it should have. Regular stainless and black oxide was available, as well as at least six different colors of powder coat painted handles.
But rumor has it the program was too much of a hassle and not profitable, so Gerber discontinued it in rather short order.
Of the colored handle versions, the yellow/black and the red/black are seen most today, as they were packaged up with a (poor quality) little flashlight and sold as a set: