Millisecond Forums

I want to make a prospective memory task in which participants are asked to press a particular key on the keyboard (e.g., Enter) every 10 minutes (while performing some other task in the meantime--here a number discrimination task). Participants can check how much time has elapsed whenever they want by looking at a stopwatch. I don't want the stopwatch to be continuously visible, thought. It should be displayed only when participants press a designated key (e.g., t, for time), and then disappear again. I also want to be able to analyze later on when participants checked the stopwatch and when they pressed the Enter key. This presents a couple of challenges for me.
The first challenge is making the clock visible or invisible. One way to do this, I thought, would be to manipulate its backgroundcolor. If that's the same color as the numbers on the clock and the screencolor, it won't be visible. Now I want to make the clock's backgroundcolor a variable, and specify on each trial what the value of that variable should be depending on the participant's response. Whatever I try, this isn't working. I can't even make the backgroundcolor a variable to begin with.
<clock timer>
/ format = "mm:ss"
/ mode = stopwatch
/ position = (80%, 80%)
/ resetrate = block
/ txbgcolor = timerbgcolor
</clock>

<values>
/ timerbgcolor = ""
</values>
...