Bar Chart Formatting Issue

Best guess: sometimes I have copied and pasted a visual and I had changed the colour for a specific value on the original visual. You could try and select 'reset to default' on the Bars option in the Formatting pane, and see if that resets your rogue customer. If it does, then the colour has been set manually at some point. If resetting messes up other settings, you can go to the Categories dropdown in the Bars option and select your customer and set the colour manually there.