Image Enhancing in R

Dear All,
I am not sure that R is exactly the right tool for doing what I have in 
mind, but a look at EBImage and rimage convinced me that this should be 
doable with R.
I have a color image and I would like to do the following: convert the 
image into a grayscale image retaining only the red color (i.e. end up 
with a grayscale image where the only surviving color is red [or any 
color close enough to red]).
Anybody has any idea about how to do that?
Many thanks

Lorenzo