After the beige boxes were discontinued, the first model released was the PowerMac G3. It was known as the
blue & white. The case was frosted white and a blue that matched the blueberry iMacs of the time. Also visible on the sides of the case was "G3".
The PowerMac G4 that replaced it was called
graphite. More grey, less blue looking. Handles were clear and case had a shine that the G3 did not. There were several models of G4 that had the same case. They were the yikes, sawtooth (agp), gigabit ethernet, and digital audio.
Next up was the PowerMac G4 Quicksilver. Very different looking. More silver looking than grey.
Lastly was the PowerMac G4 mirror drive door model. Named for the mirror finish on the drive doors. There were 3 distinct variations of this before being discontinued.
I'm not mentioning the cube as it was rather distinct and not easily confused with other models.
Hope this clears things up a bit. Check the wikipedia articles linked for more info on them. Or check lowendmac or everymac.