As I said in the other thread, your 600 seems to have a permanent discoloration. It is _not_ possible to get it any whiter, only hope is to use JiF or similar to get rid of the dirt on it, if any.
Having said that, discoloration is the most common "failure" on Amiga's, next to RTC battery leak on the A4000's. I wish I knew that 10 years ago when I placed my 1200 by my window and had it there for 6-7 years, today it's even more yellow than your 600.