If the company really want your business they should send you a small sample piece of each color, that way you can match it up.
Tell them your issue, and ask them if there is anyway they could send you a small sample of each color, they should have scraps of each that they are just throwing away, if they are making the tops onsite anyway..
Other than that I'm not sure how to know the exact color, if they have a good photo of each you can only pick which one looks the best, of course it won't be the same as seeing the material up close, as colors vary from monitor to monitor, depending on your brightness/contrast/gamma and whatnot.
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA