Whilst I have fixed this, I figured I'd post it nonetheless for anybody else who had the same problem.:
functions.php in themes/color3/ has several lines after the closing php tag, which I found causes pluggable.php to error out saying that Headers were already being sent.
This can be solved easily by going to the end of functions.php, going to the ?> tag and removing any whitespace afterwards. This should fix any problems with headers claiming to already having been sent.