UATristar's reasoning for the TED colors is probably correct, but as to why it was sitting at C13, UA
, and AA
often park their extra aircraft down towards the end of the C concourse if all of their gates are full (HP does this as well - sometimes using an extra gate besides C10). If you came in around 8am or 9am from DLH, then this is probably your answer.
Of course, if it was after that, I'm as stumped as you are
Create your own luck.