Are you able to add the color of dice you switch to using his ability as your concentrate fire roll (e.g. replace two reds with two black on a Nebulon, can you then add a third black dice for your concentrate fire or only another red?)?

Yes indeed, as the black dice are now in your attack pool - so you can choose that colour to add for Concentrate Fire.

Even without Sato you can do this with upgrades like Paragon, QBTs, Defiance and the Opening Salvo scenario.