If you're going to stay with black casings, you could always try the 1st Co in dark red robes instead perhaps? They would certainly look very striking and knightly that way, and also provide an immediate visual difference from the 2nd Co.
With regard to the white robes, xnickbananx did these a while back:
Personally I think they looked very good!
I also think that if you go with coloured robes, you might want to use a similar philosophy for the specialists (Librarian, Chaplain, Techmarine), and maybe just paint their right arm/shoulder/head in the traditional colours - leaving the rest in bone armour. You can then use the coloured robes to denote their level if appropriate (Techmarines probably wouldn't get robes at all though). Reducing the specialists' colours to these smaller areas also helps to avoid too much colour-clash with the robes and the rest of the army.
Finally, if you do go with white robes for the 1st Co, they happen to look pretty good (as well as being fluffy) on the Chaplain's black and Librarian's blue.