The background color of the bounding rectangle for text.
The following trial sets the background color of feedback message based on whether or not the response to the previous trial was correct:
<trial feedback>
/ontrialbegin = [ if (trial.test.correct == true) text.feedback.textbgcolor = white; else text.feedback.textbgcolor = yellow; ]
/ stimulusframes = [1=feedback]
</trial>