First of all it ids not cleared that image is gray scale or color? Let's say its gray scale because you want to highlight with some color. Because gray scale have dimensions, rowsXcolumns and color image (HSI color frame) have rowXcolumnXframes. Therefore, you have to create another two frames with no pixel intensity. Then you can suppose that in these two frames, you are having hue and saturation component. Now, if you have already defined some area of interest in your gray scale image, then you must be having coordinates of  all those pixels. Now you can fill Hue and saturation frame at those particular pixel coordinates. After filling the complete frames at required places, just concatenate all the frames and you will be having highlighted points with color.

Do remember that the combination of hue and saturation which you are going to use, will decide the color of highlighting.

Hope you got it.

Thanks for the guidance. 

I have grayscale images of same scene but with different intensity values. I want to represent salient regions with different colors, and rest of image using true colors. 

This sounds very helpful. However, I am sending you my idea in private message. Please have a look at it and suggest your idea based on your experience. 

Thanks