Grayscale mode is a color mode in Photoshop. In grayscale mode, images are represented using only shades of gray rather than the full pallet of colors.
In RGB mode, the color of each pixel is stored as three numbers, or channels (nnn, nnn, nnn). If all three channels have the same value then the resulting color is a shade of gray.
|Gray (80, 80, 80)|
|Gray (160, 160, 160)|
|White (255, 255, 255)|
RGB mode stores all three channels for each pixel. In this case RGB would store (0, 0, 0), (80, 80, 80), (160, 160, 160), (255, 255, 255). Notice that 2/3 of the information stored is redundant.
Grayscale mode stores only one channel called the K value. K value represents the percentage of black in the pixel.
RGB Mode to Grayscale Mode
For our demo we’ll use a two pixel image.
To convert an image to grayscale mode:
Photoshop will ask you if you want to discard the color information.
Photoshop will apply an algorithm that converts the RGB value of each pixel to a K value.
Use the eyedropper tool to see their K values:
If we use the color picker tool we can see their equivalent RGB values:
Grayscale Mode to RGB Mode
We now change the mode back to RGB
Even though the mode is RGB the colors are not restored. The RGB values are simply the RGB equivalent of the grayscale K values.
This is why Photoshop asks you if you want to discard the color information. Grayscale mode conversion is a destructive way to create black and white images because it destroys the color information.