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 colormap if you want to each to make it appear in it's original shade as explained in http://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.
v contains only one color plane, so imshow(v) treats it as a grayscale image. You have there the actual red intensities.
If you want to show the image as red, you need to do something like
v = zeros(size(i)); v(:,:,1) = i(:,:,1); imshow(v)
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