ps1scw

But why did he wear Colonel rank insignia?

Simple humility.

But why did he wear Colonel rank insignia?

Kinda looks close to Tamiya XF-54 Dark Sea Gray if I messed around with it a bit.

Thing is, Lee's final uniform is still in existence and preserved at the Museum of the Confederacy;  therefore, you can't really chose just any gray and claim accuracy. Here's an image of it. 

You might want to look up the artwork of Don Troiani, on line or in the bookstore/library. He does meticulous research for his paintings. Mort Kunsler is another very good ACW artist.

Source: http://www.moc.org/collections-archives/lee-jackson/gallery/90

And I understand most of the officer uniforms were done by private tailors so I'm not sure there was any specific CS government issue grey. They probably used whatever material they had available.

Pick one you like.  There are no color pix of ACW gray so no one can actually argue your choice is wrong.  You can Google dozens of original black and white photos and see any number of gray shades.