I'm happy for you to list any you know - worst case they're already on the to-do list, but I'd rather not miss any bugs.
How about more cut early colors?
(List is from the pre-release gt2 press disk on tcfr)
The Mini Cooper 1275S Mark I had a silver color, as seen in the December 1999 issue of the Official UK PlayStation Magazine.
The Mini Cooper 1.3 had a silver/grey color.
The Renault Espace F1 had a silver color.
The Renault Megane had a silver color.
The Daihatsu Midget II D-Type '98 had a silver color.
The Mitsubishi Pajero Mini '98 had a silver color.
The [R] Mitsubishi 3000GT '95 had a silver & yellow color option.
The [R] Toyota 2000GT had a silver & green color option.
The [R] Venturi Atlantique 400GT had a silver + white stripes color option.
The Chevrolet Camaro Z28 '69 had a silver color with yellow stripes.
The TVR Speed 12 had multiple colors (Gold, Dark Blue and Silver).
Another model of the Opel / Vauxhall Vectra that was going to be used was the Vectra 2.0i. It features the sporty bodywork seen on the Vectra GSi, but has different rims.
The Lotus Elise 111S racing mod was the normal version with a spoiler. Possibly was WIP at the time.
The Ford GT40 Race Car had a grey color, as seen in the December 1999 issue of the Official UK PlayStation Magazine and a strategy guide by Prima Games.
The first Racing Modification of the Volkswagen Golf had an extra orange paint scheme.
The RUF Turbo-R had an extra "Turbo" logo in the back, which was removed in the final version of the game. Possibly to remove any explicit Porsche references.
The Honda Prelude Type-SH had originally a blue color aswell, as shown in the Prima's Official Strategy Guide book.
The Mazda MX-5 Miata 1.6 had a white roof cover, as shown in the Prima's Official Strategy Guide book.
The Mazda MX-5 Miata 1.8 VS had a black roof cover instead.
The Lotus Elise 135 had no "135" decal on its sides