When in agenda view with column view enabled hitting 't' clears the current line so it displays as a blank line. There is no text there, so it is not a font color issue.
Though it does actually cycle the TODO value of the line. In fact, you can hit 't' again on the blank line and it will continue to update the TODO state. So it is only a display issue, and not functional.
Refreshing the agenda will display that line again.
-Rob