In a Qlik Sense combo chart, can I manually set the color of the bars representing a certain Measure using an expression?
Yes. you can do that by turning off Auto Colors in Appearance.
and then choose by expression and add your expression.
More specifically, how do I reference only one of the Combo Chart Measures in the expression? (or maybe - where can I find a reference to learn to write the expression to reference the Measure?)
I think there is no such of right now in the current version, may in future release they will provide like in Qlikview
Expressin -> Background.
But for now you can change the based on the dimension value not by Expression.
You can change the colors by dimension value for example
Appearance -> Color -> Custom By Expression.
Hope this helps you.
See my response in Re: color by expression - QlikSense
This may work for you.
You can use color but in term of condition only. Ex:- if(expression>0,Green(),Red())
or if(Year='xyz',Red()) like this.....
Another way is to create a master item of your specific measure and there you can set the color of it
then replace your measure by the master item