you can set the CSS for a line in the drop-list, on the CSS tab for that field. The entry to look for is
Drop Part:
Of course you can put an expression in here - as always - so I put in
choose(random(1,10)>5,'red','green')
and got a colorful drop-down like your screen-shot.
I'm guessing you have a slightly more clever expression, but the technique seems right.
cheers
Bruce