Controlling color of secondary X-axis

Hi folks, I'm using Metabase v0.45.3 and I'm trying to control colors on the secondary X-axis. I attached a screenshot below--there do not seem to be gear config icons on either of the fields where I can change the color:

Note that Negotiated Rate Bin is a numeric field (but I have casted it to a string to see if that helps).

If I swap billing code to be the inner dimension, I can see color pickers.

Can anyone help explain the behavior and suggest means of controlling the colors of the Bin dimension?