As far as OEM piece, working Glock pistols come in the black (or whatever you call the original polymer/tenifer color) and the OD green model shown earlier in the thread. http://www.ccfraceframes.com
makes stainless steel and aluminum aftermarket frames for Glock pistols, and I'm sure with the magic of Duracoat and other finish products you could make the slide of a Glock just about any color you want (anyone seen the XD with the hotrod flames finish...sweet!)
As far as other colors OEM from Glock, here is some information I found. I am not sure if they still make these models or not, and have never seen one IRL.
Blue & Red framed Glock pistols:
The Blue Glock Training Pistols are designed to fire color marking ammo (similar to a paintball) or rubber bullets. There are 2 models available. The 17T 9mm FX fires simunition ammo and the 17T 7.8X21 AC fires compressed air cartridge casings. Glock Training Pistols are identical to the G17 in size and shape but have a bright blue frame. They are available to law enforcement only.
The Glock Practice Pistols work normally for loading, unloading and trigger operation but are made non-firing by a plugged barrel and solid breechface. They are identical to a standard pistol in size, shape and weight but have a bright red frame to identify them as training pistols. The following models are available: G17P, G19P, G22P, G23P and G26P. Glock Practice Pistols are only available to law enforcement officers.
info from http://www.glockfaq.com