In open cv, how can i convert gray scale image back in to RGB image(color)

In open cv to remove background, using current frame and former frame, i applied absdiff function and created a difference image in gray scale. However, i would like to covert the gray scale image back in to RGB with actual color of the image, but i have no idea how to operate this back in. I'm using C++. Could any one knowledgeable of open cv help me?