When I load bulk 35 onto cassettes, I always load 36 plus a couple frames.
Load the camera normally and shoot however many frames I want that day.
If I want to change to B&W mid-roll, I wind my color film back into the cassette but try to leave the leader out for easy reloading.
I mark how many exposures I took on the removed roll onto a piece of masking tape to stick to that roll.
If I happen to wind the leader into the cassette, I use a film leader retriever tool to get it back again.
When I want to use more of the same color roll again, I load my roll into the camera, set my shutter to its fastest speed and keep my lens cover on the camera until I advance the number of exposures already used plus one frame to insure against double exposure.