Pixel Value

Basically this is the brightness of a pixel in an image, and shows the exact colour it is. This is stored in an 8-bit integer with a range from 0 to 255 of possible values. The 0 is mostly used to represent the black colour while the 255 is used to represent the white colour.

Grayscale pixel value

This is how the pixel value of a grayscale image is known, and for a coloured image the red, green and blue are separated like the grayscale image and this is known as colour planes.

The actual intensity of a colour can be found in a colour-map, where every information stored for each pixel of a grayscale or colour is an index to it because the intensity of them may not be stored accurately.

Colour Map

Reference:

http://homepages.inf.ed.ac.uk/rbf/HIPR2/value.htm