The issue of color is quite an interesting one, and it is the number focus with most customers. A 20K spread is somewhat out of the realm, but a 5K difference between a 97 Red GTS and a 97 Blue/white is quite common. This is considered by many to be the classic color combination, and even though it is a yea rolder, a 96 will usually bring even more money as the " First Year," model.
To discuss Black , the fact that a coupe was only done 2 years is not as important in that black is one of the most common colors over the years -- hence there are quite a few of them. Colors often become high demand when they were done for only one year and maybe never repeated. Vipers as a whole are rare, as previously noted, but color can make a difference and often it is just a simple case of supply and demand.
Viper Violet would be a good example. If someone wants a car in that hue, it was only done in one year so the pickings could be scarce. Sapphire Blue in the Gen IIs has a similar following, with low numbers for a single year color.
With around 27,000 built , Vipers are a unique sight for anyone -- there are just not many of us out there who own these phenomenal machines.