Your red could be someone else's green and neither of you would ever notice
Imagine a person whose colour experiences are systematically swapped — what looks red to them feels like what you call green — but who learned the same colour words and behaves identically. Locke raised this possibility in 1690; philosophers later used it to ask whether two people's behavior could match perfectly while their inner experience differs completely, with no way for science or conversation to ever catch the difference.