I would paint the the color of the existing base boards.

I have baseboard heating in my [relatively] new home. First time that I've had to deal with this issue. I don't know how high your registers are off the floor .... but ... mine are clearly part of the wall.

For rooms with hard flooring, the registers are 2"-3" inches above the floor surface and above any stained quarter-round moulding. For rooms with carpet, the registers are snug to the carpet (no mouldings, obviously, but those registers are painted the wall color too).

My current project does have some 4" Baseboards that I am painting the wall color in addition to the registers. I guess you got to size it all up and determine if the registers are part of the wall and try to blend them in -- or -- part of the baseboards (which would be difficult to match but doable). Good luck -- either way -- painting registers is a flipping pain!!!

I always matched the baseboard.