I am a B&W newbie....stack is xt2 and using the lee 100mm polyester filter B&W set shooting in raw
I'd strongly agree with posters who recommend digital filtering in post processing. This is incredibly flexible and you cannot fully undo the effect of a physical filter whereas with digital you can choose any filter in post and try them all if you want.
Lets assume I am using the green filter
Do I do a custom WB before the filter, then shoot with green color cast
WB is irrelevant when shooting RAW.
You apply (choose) white balance when you "develop" the RAW.
It is doubly irrelevant in B&W shooting in digital.
Do I put the filter on then white balance and shoot with the green color cast removed ?
If you did this (and you'd need to be shooting JPEG for this to make any kind of sense), then you'd essentially be trying to undo the effect of the filter. You want the cast the filter provides.
Again this illustrates why doing the filter in post processing is so much better IMO. You are not locked in to any choices (even B&W). The number of times I've shot B&W and then wished I could have the color version but could not, but with digital - no problem.