The side surface of a Custom Profile Beam can come from various settings:
Normally, the surface comes from the Surface setting of the Building Material set for the Beam component in the Profile Manager Palette.
However, you can override this in the Profile Manager Palette. In the Components- Selected panel of the Palette you can uncheck the Use Building Material checkbox and define a Uniform Surface for all sides surface of the Beam. Or you can set a different Surface for any of the Beam Profile's Edges in the Profile Editor Window.
Then there is the Override buttons in the Model panel of the Beam Settings Dialog. When a Complex Profiled Beam is selected, only the firs and fifth fields are active. The first button can override all side surfaces of the Beam, while the fifth button can override both end surfaces of the Beam. If you click the Override button, the Surface selection button to its right will become enabled.
I suspect you have this override button activated and this is why all you Beams have a brick surface no matter what you do.
Also check out this short clip that talks about surface overrides. The part that talks about elements with custom profiles starts at around 2:20:
https://www.youtube.com/watch?v=x9PLDkbOGx8
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