Question about Photoshop or color ysis?

I was wondering if on Photoshop or any other program that deals with images, if there is a way to yze the color values in the whole image. For example, if I took a picture, made it black and white, and saved it as a bmp, is there a feature of a program that can yze each pixel and either find the sum of the numerical values of the color on each pixel or the average? Or, is there a way for it to find the total numerical difference in color between two images? Thanks!