Switch says common wire. Is that white or black wire?
The ACTUAL question asked was: "which color goes to COMMON?" Answer: WHITE.
The only upvoted answer so far does not clarify anything about COMMON. This does not answer the question asked.
Only downvoted questions attempted to answer which color is COMMON, but are ambiguous: e.g., "white = common, neutral, or ground." Green is ground; this is not the same as common!.
White. The "common" is the "neutral" or "ground" wire, depending on the type of circuit. In normal US residential wiring, you'll have a black "hot" wire, a white "neutral" or "common" wire, and a green or bare "ground" wire.
Occasionally you may encounter a blue hot wire in ceiling fans or a blue or red traveler wire between 3-way switches.