this kind of process is possible
take my video as a example from GOD EATER BURST:
the original Caligula is blue, but i made it black and golden, especially its effect and such, considering GOD EATER RESURRECTION has a black caligula counterpart so i made it for fun, the BGM is from GOD EATER RESURRECTION as well. both are from the PC versions so i managed to import them to the psp by severly downscaling them to be more small and compressed if thats how you say it
downside:
1. you are limited to RGBA 8 bit index i believe
2. always keep the original/true image's resolution while modifying it
3. when it comes to BGM, you are limited to 512KB. above that breaks the game. So try to learn to do some sample loop
4. if you don't have a tool for unpacking/repacking a game that you want, then you're empty handed