You extracted the red channel of a color image. Each color channel is simply a 2D image with an intensity. Essentially each one is a monochrome image, and those are displayed in gray scale by default. You can apply a colormapif you want to each to make it appear in it's original shade as explained inhttp://www.mathworks.com/support/solutions/en/data/1-GNRWEH/index.html;jsessionid=523e8fb9a5651ebf4a0434578bbb but it's not really necessary and not usually done, or convert back to a color image again like Razvan did.
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