http://www.nytimes.com/2009/11/01/books/review/McPherson-t.html?pagewanted=all&_r=0
The link above is McPherson's review of Keegan's book (of which I have a copy). While an admirer of Keegan, McPherson points out some of the "numerous errors" that he says exists in Keegan's book.
I think that by Gettysburg the Sharpshooters had lost the green uniforms for blue (although with green for the NCO's chevrons, etc). I would be nice to have the choice, though.