Well I haven't seen nearly enough to qualify as an expert
but the color is usually some shade of red or orange. I remember seeing one when I was 9 or so that was a really dark red.
Having said that, it looks like this particular one (I didn't actually see it, remember) was more grayish, which is darker than normal.
http://skyandtelescope.com/observing...icle_957_1.asp