I also had to use some grays and it was easy decision, I just used Ultramarine Blue (PB29) with Burnt Sienna (PR101). They make perfect gray. There was also another color, quite strange and hard to call, something like very light brown, but gray at the same time. I mixed it using my gray and adding Naples Yellow Deep (PBr24). And for my blacks I used my great black which I mentioned about several times earlier: Perylene Green (PBk31) + Pyrrol Crimson (PR264).