I agree with Lady P’s comments. But I think this only becomes significant if the paint needs renewal or there would be an obvious difference in colour between the main building and the extension.
The real “trade-off” would be doing the sensible thing rather that than following the strict letter of the law, in order to keep the look of the building the same while avoiding unnecessary legal costs and community strife.
IMHO, the sensible thing would be to allow the extension owner to take advantage of the reduced cost of the paint job (as part of a larger project) and make a contribution accordingly.
The hard line would be to order them to make the extension look the same as the rest of the building at their expense.
If the extension would mean significantly more paint was required, I’d be strongly suggesting the put a few dollars in the pot. If not, just ask for a letter confirming that repair and maintenance of the exterior of the extension is their responsibility, then get on with it.
By the way, can we assume that the lot owners unit entitlements were increased as a result of the extension? Just asking.