You've probably thought of this already, but I think the easier solution is test the value of the expression cell. If it's zero, then colour the background. I can't find a way to conditionally highlight the dimension (based on an expression) so would be interested to hear of a solution.
For the Dimension AND Expression click on the + sign, it will expand and there is a background color option, you can use the following expression
Test Dashboard.qvw 220.0 K