greys and blacks average about 3-6m, but sometimes as much as 7-10, really depends on how much someone wants the color. The only color that generally sells for more is sigil which are very difficult to find, and even more difficult to find for sale.
Red can range around the same price as grey and black as it is also difficult to find, but bc its not as universal in its use with colors its not always as desirable.
Blues, browns and other similar colors range from about 1-3m, but can also be found as low as 500k, and as much as 5-6m depending on shard, availability, etc.