when merging in value or color mode (same for lightness and luminosity ) the sharpness derive ONLY by the BW layer (or anyway of the layer from which get the greyscale

Now you may have noticed nasty effect on the colors in both Value and color mode, that may be avoided by simply using gaussian blur on the color layer (default , 5px blur is usually good )

a example to clear:
here the Value layer on top



here the other layer, (without blurring)



Now look at the message below

Here the result of merging in value mode at 100% opacity



please note on the lower part the colors on Dali cloth, in several spot the color look ugly sprayed or pixelated and noisy, and also in the bluish area at right of his head is not too nice

Now check the difference



Sharpness is the same , that of the top layer (no so much, but unchanged) but the color is better merged with the greyscale, no more noisy or pixelated

only difference was a 5px gaussian blur on the color layer, note that (except for unreasonable high amount of blur, but not in case of blur between 2 and 5 px) color remain well separated and will not flood
in a percetible way

but the blur was quite heavy see how would look the color layer (without merging)



Note,i used 5px blur just to show that will not interfere with sharpness of combination or cause the color to mix togheter
But , no need for all that blur
i found half of that (2.5 blur) sufficent to avoid that, for me very ugly and noisy, spray of color