OpenGL - Display single color channel

Hi,
using imagemagick I'm loading image form file. Code below gives normal RGB colors.

im.read(filename);
im.write(&blob, "RGB", 8);
glTexImage2D(GL_TEXTURE_2D, 0, 3, im.columns(), im.rows(), 0, GL_RGB, GL_UNSIGNED_BYTE, blob.data());

Then texture is applied onto geometry.

Is it possible to force OpenGL to display only (for example) green channel but in all color components, I'm looking for something like GL_GGG. I want to avoid reloading different color components every time user wants to see different color channel.

regards