I've driven a couple of '99 GS400s and yesterday I drove a '00 GS300, and I could have sworn that the guage cluster on the '00 was more white than green as I saw in the '99 GS400. Is this something that really happened or just my eyes in the light I was driving? Also, aside from the engines (obviously) what are the big differences between the 300 and the 400? Anything?

 

Between the GS3 and GS4, aside from the engine, the transmission is a bit different as well (being that it's mated to a 6 cylinder engine)--it's probably just different gearing. Everything else is the same (aside from the options list).

I don't know about the Optitron between the 2 years, but you could have the brightness settings adjusted (well, your dealer can do it for you).

As for other differences between a '99 GS and '00 GS (from Lexus's CPO model webpage):

The 2000 GS 400's new modifications continue to have an emphasis on safety. Brake Assist[1] has been added as a standard feature. A Child Restraint Seat (CRS) tether anchor bracket has been added as well.

New exterior colors: Crystal White and Millennium Silver Metallic replace Diamond White Pearl and Alpine Silver Metallic.