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Emulator version: v1.5.0-dev-2540
Game version: PAL 
Bug is present on both hardware and software mode. If I drive it around the dirt for a while, it appears a purple-ish texture around the vehicle.

Software mode:
https://imgur.com/a/1a9oBmy

Hardware mode:
https://imgur.com/a/CE06DvU

Settings on hardware mode:
https://imgur.com/a/KRgwJxl

Specs:
Windows 10 64-bit
Intel i5-4570
8GB DDR3 RAM
GTX 1060 6GB

I doubt it's a bug, it's just an artifact of the blending unit. You're seeing color banding because the game is meant to be played on a CRT with dithering where the effect is less noticable.