Is there any pattern to discern, here? Like how many times each buyable car has gone through all its paints, or how many paints (other than real car paints) may be remaining?
Paint wise, first year it was just random. This year they are released according to finishes. In terms of how many paints remaining, I counted 6 possible ones that we haven't seen based on the hues and finishes released so far:
R2 Matte Metallic
Y4 Matte Metallic
G9 Aluminum Met
G9 Matte Metallic
G6 DFL
G7 DFL
In terms of special car colours, you can see the full list here:
https://www.gtplanet.net/forum/threads/gt-sport-special-cars-color-variation-list.381851/
In terms of what has been officially buyable though, the list looks as follows:
M4 SC (White, Blue, Cream, Orange, Red)
AMG SC (Grey, Orange)
C7 (Blue, Silver, Red, Yellow, White, Dark Blue)
Evo (Red)
STI (Grey, Yellow, Blue, White, Silver, Red)
Mustang (Orange, Blue, Yellow, Black, White)
Mazda (Light Silver)
RCZ (Blue, White, Aqua, Black, Silver)
Alfa (Yellow, Red, Black, Met Red, White)
As you can see the Safety Cars have exhausted their colours a long time ago. For the others, it seems with each rotation we see a different colour EXCEPT for the Evo and Mazda which are only available in 1 colour so far. Though as someone has noted, we are currently stuck at Subaru...