What I did on my project is someone sent me a bmp file of a 34 Ford 3 window. I then went online to paintscratch.com and looked at the color I was considering, which looked nothing like the color in real life. I then adjusted the color setting in the paint program and colored the car in the bmp file. It looked fairly well, but nothing what the paint I chose looks like. I do not think there is any programs that exist except for HOK. There is just too much color variation in computer monitors to make it reliable IMO.
What Martinsr said about finding a car with a color you like is the key, worked for me.