My initial thoughts were on the decks I already had built and what another color could do to them...
* Purphoros (which I'm unretiring and retooling)- man, adding white for
Smothering Tithe and
Anointed Procession could do a lot in this deck, but then again so could green for
Doubling Season and
Parallel Lives.
* Karador Shadowborn Apostles would love red to add in some of the RB demons such as
Sire of Insanity just to give the deck a different feel (I ran Saskia as the commander at one point but the deck ran less smoothly as Saskia really didn't add anything other than the color while Karadors ability at times comes in handy).
*
Grand Arbiter Augustin IV would love some black. Give it more removal and some of the depth black has with sac abilities to add to it would be insane, and my playgroup hates this deck as it is.
As for adding a color to a general I haven't built yet, CleanBelwas already hit the nail on the head.
It's not a good choice as such, but I'd add green the The Scorpion God. I'd love a Jund -1/-1 counter deck.
I just drooled a little.