There is no reason why this should not work.
You can set the Color property for the Text box , and everything should be fine.
=Switch(Fields!My_column.Value > 6, "Red" , Fields!My_column.Value < 5, "Black" , true , "Yellow")
gives:

What does your baseline data look like? Is it 6% "6" or is it really "0.06"?
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