Perfect, thanks Dan! That's exactly what I needed and it works pretty well.
For others, to take out your "shirt color" from the image easily, you can just subtract all your print layers from a shirt channel, and then use that for a mask on your merged image and then save that as a PNG so you can use your actual separation file as a mockup with no background.
Attached some screenshots of the shirt channel where the layers are subtracted out, and then layers where that channel is used for a mask in a merged file.