Colour pixels are different from grayscale pixels. Colour pixels are RGB, meaning they have three pieces of information associated with them, namely the Red, Green and Blue components. Grayscale pixels have one component, a gray tone derived from a graduate scale from black to white. A colour pixel is generally 24-bit (3 × 8-bit), and a gray pixel is just 8-bit. This basically means that a colour pixel has a triplet value comprised of 0..255 for each of red, green and blue components, whereas a grayscale pixel has a single values 0..255. The figure below compares a colour and grayscale pixel. The colour pixel has the R-G-B value 61-80-136.The grayscale pixel has the value 92.

It is easy to convert a pixel from colour to grayscale (like applying a monochrome filter in a digital camera). The easiest method is simply averaging the three values of R, G, and B. In the sample above, the grayscale pixel is actually the converted RGB: (61+80+136)/3 = 92.

Now colour images also contain regions that are gray in colour – these are 24-bit “gray” pixels, as opposed to 8-bit grayscale pixels. The example below shows a pixel in a grayscale image, and the corresponding “gray” pixel in the colour image. Grayscale pixels are pure shades of gray. Pure shades of gray in colour images are often represented with RGB all having the same value, e.g. R=137, G=137, B=137.