Question: [Wednesday, 18th Tamuz 5781]
May one paint their nails during the three weeks?
It is permitted to do so throughout the three weeks, although some are stringent starting from the nine days.
Explanation: There is no listed restriction against painting ones nails during the three weeks, other than the issue with cutting the nails, which is only restricted during the week of Tishe Beav according to most Poskim. Nonetheless, some are accustomed to diminish in makeup and jewelry throughout the nine days, and seemingly those who follow this custom should likewise not paint their nails during this time.
Sources: See regarding diminishing and jewelry and makeup: Ashel Avraham Butchach 551