After you’ve done a pretty spray job on your car using the colour sliders, saved it and exited out, is there a way to get back the values of those sliders so you can do the same colour on different car?
Cheers 
If you’re in the same game session (ie, you paint, save, go race a while and come back) the color will be among your previous paint colors. Just use the RB button to get over to the last tab where the most recent colors are shown.
If you’ve completely exited the game and come back, there are a few more steps you need to complete first. You need to start off by choosing the car that you’ve already used this color on. Act as if you’re spraying it again, but this time, choose the same paint type (Normal or special) and choose the active color on the far left. This will add the color to your previous colors again so that you can follow through the steps I mentioned above.
I’ll have to check again (I’m not in front of the game right now) … but I’ pretty sure the method you describe of finding a color when not in the same session does not work on any special colors. Pretty sure when you come back to the special colors palette later they always start with the same default color settings.
True, the default special color offerings don’t change. But I’m not suggesting you rely on them alone. Let me try to illustrate my process here.
The color I originally used on this Porsche was a custom two-tone polished – red base (H=0, S=1, B=1) with a white highlight (H=0, S=0, B=1). In the small preview chip, it shows as red over white. I saved the paint to the car and exited the game.
After re-entering the game, I chose the Pacer to apply my custom paint to. Sure enough, my custom color was not available in the previous colors, nor had my special colors changed from the boxed standards.
So, I got back in the Porsche, and acted as if I was going to repaint it. Without making any selections, my custom color was already showing up as the current color. (I previously posted that you need to select the same type of paint, but I was wrong. My custom color even shows up when under the normal paints.) From here, I just pressed “A” to confirm the current selection.
I got in the Pacer again, went to paint, pressed RB to get over to Previous Colors and there’s my red over white paint chip.
Now my Pacer can match my Porsche. 
Cheers both, it might help with what I’m after. What I’d really like is the colour sliders again with the colour already “numberfied”.
Thanks!