Okay, you're all a little right, and all a little wrong,

CMac - You cannot boot a G3 system with a leopard disc, its not a supported processor.

all - System specific discs(gray discs) will only work in the specific system they are designed for. not only can you not install the system, but as mentioned - and error message will pop the second it boots alerting you that you can not use this disc with this machine, no disc utility, no password reset - do not collect $200

System specific discs for intel are not universal binary, therefore attempting to boot a PPC machine with an intel disc will result in a Kernel Panic. however a Retail 10.5 disc will boot both PPC and intel (processor permitting - G4 867 and Up)