I think the yellow deflector dish is because the deflector was starting up. It becomes blue once the deflector is at its normal operation.
IIRC — and this is going to some discussions that go way back, and may not even exist on the board anymore — but we established that it's fairly consistent within
The Motion Picture that the dish is only blue when they're at warp speed; it's the orangey color when they're stopped, or traveling on thrusters or at impulse speed. I know that seems a bit confusing because of so many scenes where the
Enterprise seems to be plodding around or through V'Ger, but that's because it's moving at warp speeds relative to the ones V'Ger is traveling at.
It's only when you get to the subsequent films that the dish is blue all the time (except in those scenes in
Wrath of Khan where they recycle effects footage from TMP).