Global variables not accessible in color setting of Tomorrow Forecast Icon

I put the same formula into the color setting for both some text and some icons (Tomorrow Forecast, 2 Days Forecast, etc) - and the color setting works perfectly for the text, but gave errors for the icons. The errors were from mu and ce functions, caused, I believe, by gv references returning nothing.

Trying to simplify the problem, a simple reference like


doesn't seem to work in the calculated color setting of these icons.

Apologies in advance if I've stuffed up!