Color communicates a ton, without saying a word.
Sometimes the bindi is a symbol of religious affiliation, or a mark of a recent religious ceremony; sometimes the bindi is merely for beauty. It's also said to retain energy in the human body. But while anyone (including men and children) can wear bindis, only married women (excluding widows) may wear sindoor in her hair part.
Good to know! I always like to know the origins of these colorful traditions. Any other color symbols you are curious to learn about?