A: Correct: if your only piece in a system is red, and you sacrifice it to attack, you will have no ships left in that system to attack with. Ship/s in some other system/s will have to be the ones attacking.
The same would be true if you sacrificed your only green ship for grows: you will have no ships left in that system which can grow. You will have to choose ship/s in some other system which will use the grow/s gained by the sacrifice.
If someone attempted this with a blue or yellow sacrifice, it would be obvious that there’s nothing left there to change color (in the case of blue) or move (in the case of yellow).