The bone white look was unfortunately only a post-heresy change but there are several inconsistencies in their depiction you can take advantage of.
Firstly, if you look at their art from the
HH: Collected Visions, you can see them depicted in a color that's not really black.
(large image)
A convenient fluff explanation can be found in
Unremembered Empire, since it makes mention of a shade of black from Caliban called
calibaun so maybe this is what it could look like. I've copied the setence here:
Astelan is also depicted with a green shoulder pad in that picture, which is mentioned in the
HH series. (Not sure which novel or short story, but its mentioned that they paint their shoulder pads in the colour of Caliban's forests - this is before the schism).
Another way you can mirror the bone white colour is to paint their shoulder pads, as well as hoods and tabards white, as seen in their depiction on the
Descent of Angels cover or in the cover for
Prince of Crows:
Or you can just do your own thing since there's no real consistency. As example of this is the cover for
Fallen Angels or this image from the
HH: Collected Visions book (which has them in green/gold):
AND then there's this guy, who has the coolest looking models ever (this is without using the
FW models! all that green stuff!)

:
more here:
http://z15.invisionfree.com/The_Great_Crusade/index.php?showtopic=4030
http://s200.photobucket.com/user/desmondhng/library/PreHeresyDarkAngelBlog?sort=3&page=2
He based them off the look in
HH: Collected Visions. Check out his models, he's got the
DA pre-Heresy look completely covered. Has the best looking model for the Lion and Luther has well.
Anyway, hope this was helpful!