heres this site, you can put in any steed you want to make and at the top right it tells in 3 numbers for a nightmare
its 140 - 122 - 33
meaning 140 black, 122 maroon, and 133 white. seems a little harder then the other guys way
but heres the site you can make any steed you want to
ive never tried it but other people i know have and say it works
theres also this page that shows the formula the other guy put up + a couple other forumlas for nightmare and every other kind of steed
and yes it does starscream