Differences in Blown Highlights and Crushed Shadows between M246 Mono and M240
I don't use the (A) mode and Auto ISO when I shoot on the street, because the lighting condition on the street is unpredictable, and because the camera's exposure metering is not optimized for what you need. So constantly, while I'm shooting on the street, I am manually adjusting my exposure for the next shot, based on the changing light, and also the balance of light and dark, relative to the subject, which can confuse the automatic metering system.
Usually, I'm pretty good at it. But there are times when I miss the exposure. This is especially true when I forget to adjust the exposure setting when an unexpected photo opportunity pops up, just as the lighting condition changes from one extreme to another. When this happens, the exposure difference could be as much as two stops. And if you fails to meter correctly, it's then up to Lightroom to salvage the image, assuming that you shot in RAW DNG.
Of course, what I am saying is true for all camera manufacturers. Human error in metering will happen if the photographer shoots without electronic metering aids. However, what I do see in Leica M Bodies do seem to follow a consistent pattern. Highlights are prone to becoming blown on the Monochrome bodies if the highlight of the image is overexposed by one stop (in strong highlight situations), while shadows are "crushed" or blocked on the color M bodies, if underexposed by more than two stops (in lower light situations).
By contrast, I also noticed that the Monochrome bodies fared better with shadows when underexposed. If the image is underexposed by one and a half stop, usually the shadows can be recovered in Lightroom. If underexposed by two stops, you're pushing it, but you should still be able to use the image after post on Lightroom. If more than two and a half, then the image is likely to have significant crushed shadows.
I am aware that the image files on the Monochrome Leicas cannot be saved if underexposed too much, especially in low light situations. However, what I have come to understand is how much better the Monochrome sensors are in low light situation when compared to the color M Leicas. It's almost as if Leica had made it this way. I mean, just look at it - the M 246 Monochrome has an extra stop of usable film speed, for low light conditions, while the M 240 can stop down to ISO 100 (pulled) for better image quality under optimal light conditions.
It thus makes sense to use the Monochrome bodies in lower light situations, versus the color M Leicas. For the M 240, the use of a faster lens, like a Summilux is necessary in low light conditions, while a Summicron is all that you need for the Monochrome bodies. As such, the usage of lens predetermines the types of images that one would take for either camera body. For the Monochrome and Summicron pair, having deeper depth of field is more likely, whereas the color M Leica and Summilux pair would likely isolate the subject more, in an effort to render background bokeh. For that reason, I generally recommend the 35mm Summicron on the Monochrome and the 50mm Summilux on the MP 240 for those who carry both bodies. If just the one body, then the 35mm Summilux.
A final advantage that the Monochrome bodies have in poor lighting conditions is an inherent benefit of black and white photography - you don't have to deal with the suboptimal color balance distinctive to low light situation. Immediately monochrome photography deals away with that problem by simplifying the image from color to a gray scale. And if the image is underexposed significantly shot on the Leica Monochrome bodies, the resulting crushed shadows after repairs on post might appear grainy and acceptable for use.
As for color M bodies, it does appear that the M 240 does handle blown highlights significantly better than the M Monochrome. From my experience, one can save an over exposed image up to one and a half stops without encountering significant problems in loss of image details. With that said, bright light conditions are normally not a problem in photography, given that there are so many more ways to overcome it without adversely affecting the image quality - whether by stopping down, or using a neutral density filter.
Bottom line - I believe the Monochrome M bodies excel under suboptimal light conditions while the Color M bodies excel in extremely bright conditions.