iMessage not working for one contact

Hello, Malteser1471A.

Thank you for posting in Apple Support Communities. We understand that you’re having trouble getting an iMessage sent to a fellow iPhone user, and you’d like some assistance. We may be able to help! It sounds like you’re able to send them a message, but it sends as an SMS (green) message instead of an iMessage (blue), and you have both their iCloud address and telephone number saved in Contacts on your iPhone. 

If we’re on the same page, a few things could be happening. 

• Your friend may not have their number set up for use with iMessages. We recommend referring them to this article for assistance:

Add or remove your phone number in Messages or FaceTime - Apple Support

• Both you and your friend will need to have an internet connection by either Wi-Fi or cellular data when you send the message. If either of you are out of range the message will default to an SMS message. 

This article explains:

About iMessage and SMS/MMS - Apple Support

• It’s possible that your conversation history is remembering your last connection type. Try starting a new conversation and make sure you’re choosing the best contact option. Ideally, you’ll want to see the number in blue indicating an iMessage connection. 

Warmest Regards.