PlayStation 1 (PSX) Hardware Versions Have Graphical Differences

Look at the fog. Comparison of a SCPH-5501 (US, from around October 1997) and Japanese SCPH-1000 original PlayStation from January of 1995. Some early US and PAL model PlayStations also have the same graphical differences, but it's harder to tell which ones without turning them on.
All PU-7 motherboards and some PU-8's use dual ported VRAM which can be read out of by the video output circuitry at the same time as it's being accessed by the CPU. This dual ported VRAM was expensive and Sony switched to cheaper and more common SGRAM in late 1995 or early 1996. They had to do major changes to the GPU to accomplish this, and one change that they made was increasing the bit precision of "Gouraud shading" from 5 bits to 8 bits. Final Fantasy IX uses this shading for its overworld fog/mist special effect. They also fixed a lot of bugs which means that usually the SGRAM revisions actually perform better, even though you'd theoretically expect dual ported VRAM to be more performant if the GPU's hadn't had any bugs fixed.
This is why there are both blue and green colored debugger playstation units. Blue ones had the older GPU and dual ported VRAM; green ones had the newer GPU and SGRAM. The older GPU is much, much less common if you buy a playstation used. Only some the ones with RCA jacks have them - but there are also ones with RCA jacks that use the new GPU!
00:01:00 - start game on newer GPU.
00:04:18 - start game on old GPU. Note the "banding" in the fog.

[@weegeesoul]

Interesting comparison. I have a PAL SCPH 1002 which has the new GPU and a Japanese 1000 with the old GPU. I also tested some games on both systems and the old GPU indeed has some color banding in a few places.
Now I wonder if there are any framerate differences between the two models.

[@vedrisnightmare]

Could this be due to age?