In Android Messages, a purple dot next to a contact generally indicates that the message is being sent as a Rich Communication Services (RCS) message. RCS is an upgraded protocol for SMS that allows for features like read receipts, typing indicators, higher quality media sharing, and more interactive messaging experiences.
If you see the purple dot, it suggests that the messaging app is utilizing RCS for that conversation, assuming both you and the recipient are using a compatible messaging service that supports RCS. However, if the recipient does not support RCS, the message may be sent as a traditional SMS instead.