So the DAC (AD7302) I ordered arrived - but it turns out digikey sent me the wrong chip :(  It was in the correct packaging - but it was actually an ILD55.  So that was unfortunate, I contacted customer support and I'm still waiting for a reply.

Meanwhile, I found a different DAC on Amazon with free (Prime) shipping, so I went ahead and ordered it: AD558.  It came today so I wired it up.

And boom! Now I have my 16 unique color bars!  These are 4 bit color in the form of R, G_HI, G_LO, B

The green color goes through the DAC so I can convert it to 3 unique voltage levels.  Unfortunately where those voltage levels shift there is some bleeding.  You can see it in the pic after the first 3 green bars, it goes to light blue just before dark blue.  As best as I can tell, the DAC can take 200ns to shift voltage levels, This equates to 0.2uS - and the pins coming straight from my ATMega take only .125uS - I believe this is a large enough discrepancy to account for the brief bleeding. 

So... now I need to think about this and do some research.... I'll post back when I come up with something!