Board Thread:General Discussion/@comment-32182236-20181114014909/@comment-32182236-20190621142020

The names turn yellow when monsters spare us. A Froggit explains that sparing is saying you don't want to fight.

So, when their names turn yellow, they told Frisk (or Chara somehow) that they do not want to fight.

This is why Sans' name turns yellow in his fake sparing segment, and why Toriel's doesn't, even though she never truly wished to kill us.

Another Froggit says it is the monsters who change their own names to yellow, because when you ask to disable it, he says that he'll tell everyone to stop using it, and when you ask for them back, well, too late, they're already thrown away... But we do have pink names..