ug, if your starting image is monochrome, then you cannot determine the original color. You cannot. Not if all you have is the grayscale image. Period.
You can convert a grayscale image into an RGB image using cat():
rgbImage = cat(3, grayImage, grayImage, grayImage);
of course it will look all grayscale even though it is a "true color" RGB image - it's just that all the "colors" are gray. You can apply a pseudocolor lookup table to the gray values, where each gray value gets mapped into some color, to get a multi-colored image, like this:
rgbImage = ind2rgb(grayImage, jet(256));
however the colors are NOT the original colors of the iris as if you had snapped it with a color camera.