4.19 added a new material node that should help here, the Channel Mask Parameter. Just realized this didn’t give an visual example in the release notes, but it’s something like the following:
https://mail.google.com/mail/u/0/?ui=2
Note how the mask channel value is set to green, so it pulls the matching data (in this case the quads containing data in the green channel).
Internally it’s just a wrapper for a vector parameter, so you can swap it at runtime or in an instance without creating a new permutation:
If you don’t want to update engine version, you can do the same thing with a vector parameter going into a dot product, just set the vector to the color of the channel you want to extract. This was introduced primarily to help with material layers where masks and blends are reused and we need to avoid compiling a separate shader for every variation.