I have an image which is just made of one color ? (it could be 1x1 image or 1900x1200 image is still one color)

How do I know which color is ? 

For instance you could think the image is black but then you would learn that it's really
"020201" 

or maybe you think the image is some kind of red but I Need to know if it is "FF0000" or 
"EE3030" and so on...