How to Calculate size of colour images ?

In color images each dot (please refer to black and white image calculation if you are not familiar) can have N number of colors. We hear 256 color images, 16 million color images etc. That means to represent the dot you need more than one value, actually more than two values if you compare to black and white. To represent more than two values what we do is to assign 8 bits or 16 bits per dot.

In case you use 8 bits, you can represent 256 states for the dot, which we can use to represent 256 different colors. in the same way allocating more number of bits you can represent more colors. as an example 16 bit image depth can display up to 2¹⁶ Colors (65536 Colors) and with 24 bits 16 million colors.

So the file sizes of images become a multiplication of color dept. Hence a 256 color image will be 8 times the black and white and 16 million color image will be 24 times a black and white image scanned with same DPI parameter.