If you have a "grayscale jpg image" then that could mean that you started with an RGB image in which the R, G, and B channels were all equal. Manipulating such an image could potentially end up with unequal channels.
The alternative is that you had an indexed (pseudocolor) image whose colormap consisted entirely of gray entries; if so then the color the image would show up would depend upon the colors available in the colormap you are using.
Please check the size() of the image: it would have 3 dimensions if it is a truecolor image whose R, G, and B were all equal.
