Color and B&W are indeed different worlds. One meaning for our digital camera use is this:
Color work has three data color channels, red and green and blue (in our RGB system).
Grayscale has one gray channel, called Luminance (grayscale brightness).
Luminance is a computed number from this NTSC television formula: (B&W cameras recording color scenes and converting for B&W TV)
Luminosity = Red x
0.3 + Green x
0.59 + Blue x
0.11.
The idea is this is how the brightness of colors look to our eye (and B&W film), green looks bright and blue looks dim, etc. And the sum of 0.3 + 0.59 + 0.11 is 1.0. This is how B&W TV tries to reproduce the same visual brightness, called luminance. This is how the shade of red lipstick comes out looking about right in B&W pictures. Older orthochomatic film shows the lipstick as pretty much black. Panchromatic film is about showing these shades correctly. Old hat in recent years, but a very big deal in older days. Anyway, this formula is about that effect.
But if now RGB (255, 101, 80), (red is clipping), this comes out in mono as:
Luminosity = 255 x
0.3 + 101 x
0.59 + 80 x
0.11 = 145.... the
Sum is only slightly above mid-scale, far from indicating clipping, and only slightly above middle brightness in B&W.
Data in all three RGB channels are reduced in this proportion. It is a mathematical abstraction, and is NOT the actual real data values.
This reduction hides clipping, values are no longer at 255. But the RGB sensor was clipping.
So if we look at the single gray histogram in our camera, we see value is 145, and we think all is fine in the world, just middle brightness.
But in our RGB sensor, the red channel is 255 (and probably clipping), and it needs attention to exposure. It does not affect B&W much, some loss of detail, but there is no red color to change color value (in grayscale). It is mainly a brightness thing for B&W.
So bottom line, we should always look only at all three RGB channel histograms.
Surprises in the Use Of Histograms