Well technically the molds in question were not called spychangers until the 2001 toy line: Robots in Disguise (RID). In Generation 2 they were called "go-bots" in reference to their ability to "go" fast on Hot Wheels scaled tracks. I was poking around the Transformers Wiki today looking up info on the Cyberjets, when I stumbled upon a variation I had not yet memorized in my extensive knowledge of the spychangers. Since Firecracker's body was translucent red, and Blowout's was translucent smoke, some of the body parts of wave 1 Gearhead, and Motormouth, namely the underbody, were also translucent red and smoke. In wave two when Optimus Prime and Megatron replaced Firecracker and Blowout the underbody on Gearhead and Motormouth changed to solid red and silver.
This was due to the cross-pollenation of molds. To increase the color variety amongst these bots different elements came from different plastic injection mold-trees. The shared trees between the above mentioned cars became cast in different colors as Firecracker and Optimus are the same vehicle, to differentiate them Optimus was a solid red as opposed to Firecracker's translucent red, and Megatron was silver, as opposed to Blowout's translucent smoke. So when the wave 2 bots were cast, a color change occured on both Gearhead and Motormouth who shared elements on the other cars' plastic mold trees.
Although today's topic is very nerdy, I think it's pretty nifty.