The standard deviation of monochromatic frames

there is an article which i'm trying to write the code of it in one step of the code, it mentioned "computes the standard deviation of the frame feature vector. (The standard deviation of monochromatic frames is equal to zero)" i do n't even understand it the step before it is calculating the color histogram in hsv color space which i've done it can s.b please tell me what does "standard deviation of monochromatic frames" exactly mean? thanks alot