This would be nice indeed!
I tried several complex color expressions to "trick" a different coloring per expression, but no luck.
Strange that a custom color expression applies to all the expressions, while the option "Multicolored" for a combo chart does differentiate automatically... They should make coloring a property of the (individual) Y-axis.
Coloring by expression differs from the behavior when using a single color in more cases.
It doesn't make sense to me.
See this line-chart, where I use an expression to color dates before the 1st of March. Then all of a sudden the area coloring changes from a distinctive fill to a very transparent generic blur...
And the color function "BLUE()" is not the same blue as the blue from the single color optionbox!
And the 12 colors in the Combo chart's Multicolored schema are different from the 12 colors in the line/bar chart's single color optionbox.
Using consistent coloring, for a specific measure, throughout the application doesn't come easy.