And don't forget the contrasting colors like wheels, chrome trim, tires, door caps, etc. You may be making too much of it. Find a color in a book you like, or print out photos from the internet (use paper, not a computer screen) and take that photo to your painter. Tell him you want the color to look like that in the sun.
Better yet, enlarge an area of the photo to 8x10 and hand it to him on glossy or semigloss photo paper. Then when the car is done you can hold that paper up to the car and make sure it matches.