This is an old post but there is no answer provided so I'll share what I have found.
When you have one expression on the bar chart the multicolour option is available. When you have more than one expression the multicolour option greys out.
I have a chart that I want to fast change between bar chart and table. In the table I want to show two expressions in the bar chart I want to show one and for it to be multicoloured.
To make this work I had to remove one of the expressions, switch multicoloured on then add the second expression back in and switch if off for bar chart. This leaves multicoloured turned on but still greyed out.
Sadlly for your issue the bar chart with two expressions will not show multi colour, it shows one colour per expression