Don't you still have the original rgb image? If not, why not? Once you throw away color information you can't get it back unless you've somehow saved it. Like, you could do
grayImage = rgbImage;
but why? If you do that you still have the RGB image so what's the point of stuffing it into your grayscale variable?
If you want to assign different colors than the original, you can use ind2rgb(), but it doesn't sound like you wanted that.
