Sending message, they show up in red I've been messaging with someone from my iPad to their phone for over a year, but now they show up in red. My messages cannot be sent. 
I know it's not on their end because it happens with other people too. 
How do I correct this?
Double check they have iMessage activated in their Settings. Sometimes changing SIM card when travelling can remove a number you have been using for iMessage before, or people don't reconfirm it afterwards.