Customize one visual only theme

jcastr02 

You should be able to use conditional formatting to override the theme colors.

First, we will need a measure that read a value from the dates table to determine what shade of red to use.  In mine I use the month number (1-12) to pick the shade of red.

Bar Format = 
VAR _Month = MAX(Dates[Month Number])
RETURN
    SWITCH(
        _Month,
        1,"#ff0000",
        2,"#ffa07a",
        3,"#f08080",
        4,"#fa8072",
        5,"#e9967a",
        6,"#ff6347",
        7,"#cd5c5c",
        8,"#ff4500",
        9,"#dc143c",
        10,"#b22222",
        11,"#8b0000",
        12,"#800000"
    )

It is important to make sure the format of this measure is set to Text.

Then, it the data color for the bar chart go to conditional formatting.

And set the formatting to Format By: Field Value, Based on field: Bar Format.  'Bar Format' is the name of the measure we created with the color switches.

And here is the result: