Wow! That's about the most complicated, convoluted way of converting a gray scale image to RGB I've ever seen. If image_newC is a unint8 gray scale image, you can simply do
rgbImage = cat(3, image_newC, image_newC, image_newC);
If you want an RGB image with some colormap applied, just as jet(256), do this:
rgbImage = ind2rgb(image_newC, jet(256));
Please see my attached demos if you want to colorize a portion of the image but not the whole image.
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