How do I only add an alternate material to the top support layer and not on the bottom layer (shown in purple)? I set bottom interface layers to 0 but it still generates it.
Help! Does anyone have any suggestions? Thanks.
To help speed up the chances of getting a useful reply, you might want to upload the 3mf file to the forum. It’s not a guarantee, of course, but it often makes it much easier for others to take a closer look and provide a more accurate and faster response. 
Sorry, I do not know if that is currently possible.
But you may be able to use a same material type support. If it is OK at the bottom, why not at the top?
Alternatively, use tree support from the build plate only.
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I think this is a Studio bug. I made a similar but larger shape to test, and Studio always includes a bottom interface layer. The same settings in OrcaSlicer do not produce a bottom interface layer.
Bambu support.3mf (30.7 KB)
However, I would favor base-plate-only tree supports. Normal supports with no interface on the bottom will likely scar the print.
I found another bug in the filament preview when allowing automatic support of the entire channel. When checked, Avoid interface filament for base produces a single line of interface(yellow) on the bottom slope. Unchecked, all the lower support layers alternate between layers of interface and default(green) material.
The line type preview correctly shows interface material only along the slope. I do not have an AMS, so I cannot make a test print to see the actual result.
Thanks for responding. Yeah, I started trying different configs and it does seem to be a bug. I will support it some other way, but yeah, in this case, good idea, supporting off the build plate makes sense. Although I may print the part at 45 degrees for strength reasons, in that case I’ll likely design my own support for the opening.
Edit: Also, I don’t want alternate material on the base because it does not stick real great (desired result and why I’m doing it) and don’t want the entire support moving.