New answers tagged black-and-white
After making your first picture black&white (1 bit per pixel, or 8 bpp grays...) promote it back to RGB (24 bpp) or whatever mode is the inserted picture. This way the inserted picture won't be be turned to B&W.
With the current version of Lightroom this is not possible, however you can create a smart collection of images based on their Treatment which can be either color or black and white: If you can find a way to set your black and white photos to use this Treatment flag to indicate they are monochrome (again, I can see no quick way to do this), then you ...
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