If the red channel is the only one without the felttip marker present, then you're pretty much stuck with a grayscale (B&W) image. If you try to replace the green and blue channels with the good red channel, then all of the channels contain the same information - resulting in a grayscale image.

Is it possible for you to post the image here? (Make sure to keep the file size of the image under 100KB or you won't be able to attach it.) We might be able to give you some other possibilities for fixing the problem if we can actually see it.
You might also take a look at this tutorial to see if there's anything there that might be helpful to you.
Jeanie
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