hop-up unit is tough, TM bo-mar rear sight, a few small springs, those are the toughies.
Everything else can pretty much be upgraded with an aftermarket piece.
The hi-capa's colour schemes are generally very limited to silvers and blacks, but you can get different finishes like a matte silver or a mirror finish like RacingManiacs. Other colours that will define your pistol is the plastic grip, shooters design and WE make quite a few colors ranging from black, blue, purple, grey, clear, OD (tanio koba grip), etc.
Midframe wise the only other colour from matt silver, shiny silver, and black is Tanio Koba's kimber mid frame, which comes in OD. although those are extremely rare to come by now. I think Illusions manufacturer can anodize their parts to gold if you really want that bling look.