There is no color aluminum, it varies depending on the type of aluminum, how clean or dirty it is, whether or not it's polished etc. It also depends on what's reflected on the metal. Put some polished aluminum in a red room and it will have a red cast to it. It's more of a matter of choice in this case.
Perhaps people could post their RGB colors for bare metal?
There is also a post here that it worth looking at-
http://www.simmerspaintshop.com/foru...t-chiping.html