Every image in Photoshop is made up of individual channels, which are found in the Channels palette. A Grayscale image has one channel while RGB and Lab images have three individual channels. A CMYK document, on the other hand, has four channels. The channels that make up the RGB and CMYK documents are called color channels. Each channel can either have 8-bit of brightness information (256 shades of gray) or 16-bit (65,536 shades of gray). In an RGB document, each color channel is distinct from one other.
In the general sense, the “Red” channel tends to contain the greatest range of contrast values in an image compared to the other two channels. By contrast, I mean the highlight areas are much brighter and the shadow areas are much darker. On the other hand, the “Green” channel is where the high-frequency details in an image can be found.
The “Blue” channel is perhaps the least useful of the three channels. Most of the defects of a scanned image, for example, like scanner noise, film grain, dust and scratches are found in this channel. In a digital image, it is the depository of the luminance noise and sensor’s dust.
When an RGB image is directly converted into “Grayscale” mode using the “Image > Mode > Grayscale” commands, the three-channel document is reduced to a single channel document. In the process of removing the colors, Photoshop extracts a majority percentage of green (for detail), a small amount of red (for contrast), and still smaller percentage of blue. Because not all images have the same content, the result is not always what you want.
Another factor to consider when doing the conversion to B&W is the luminance information in the individual channels of an RGB document. Essentially this means that when, for example, the color red exists in an image, it will appear as light gray in the red channel and dark gray in the other channels. Therefore, if you have a red rose lying on a white lace mantle, there will hardly be enough contrast in the red channel. It’s the same thing when your image is composed mostly of white clouds and blue sky. In the blue channel, both the clouds and sky contain white to light gray pixels. On the other hand, because of the absence of blue in the red channel, the blue sky will appear as dark gray in that channel, providing a greater contrast to the white clouds.