Not enough to worry about if you are considering a black roof for cosmetic reasons. Once the asphalt base is heated, that is what is transfering the heat. Asphalt heats up and holds it. A lighter color is going to reflect some, but a miniscule amount. For a cooler attic, there are materials such as radiant barrier along with proper venting that contribute to a cooler attic.

radiant barriers - -hmm. That might be good to explore

In field tests of different roofing materials and attic venting it was found that the only factor that made a significant difference in the temperature of the shingles was the color. The lighter color the cooler the shingles so it's hard to say how effective grey would be.

A radiant barrier will reduce the heat transfer to the attic floor and so will insulation.

I was looking at Timberline Cool Color Shingles. I haven't seen them in real life yet. Anyone with opinions? Are they a gimmick?
Not sure I want to sacrifice appearance for energy savings. . .

http://www.gaf.com/Roofing/Residential-Roofing/Roofing-Products/Shingles-Styles-Shingle-Colors/Timberline-Shingles/Timberline-Cool-Series-Shingles/Timberline-Cool-Series-Shingles.asp#

Here is a link that might be useful: Cool Series Shingles

Ventilation has little effect on the heat from the underside of a roof because radiant heat energy (infrared) is transmitted through the air without heating it and is then absorbed by the attic floor and the floor then heats the air in the attic. Ventilation helps but it's primary purpose is to help moisture to escape. Excessive ventilation can introduce unwanted moisture.