The various overlay modes are just different ways to preview the SELECTION of the mask. They have nothing to do with the adjustments that are being applied.
To switch between seeing the mask / seeing the adjustments being applied by the mask, press O key. In any case when you exit the local adjustments tool, you will always just see the adjustments.
You have always been able to use e.g. a red semi-transparent overlay - and to change to e.g. green for the mask overlay, if e.g. it is a photo of red flowers and a red mask is hard to distinguish. The photo is not being made red, or green, when you do this. The new software includes some new mask preview options. One of those is to present just the mask in shades of grey, without showing the picture underneath at all. This does not mean the photo is being made B&W - you are currently not seeing the photo at all - it is just another way to see the extent and degree of the mask's selection.
If you want a B&W photo treatment you will need to use the global settings to do that, and then to use whatever local and global adjustments best achieve the tonality that you want.