[R] cell counting in R

[Doug Bourne]

I have a bunch of jpg files that I need to count cells on.  Some cells are green and some cells are red.  Is there a function like locator that I could use to manually count the cells?  I would like to be able to click on a cell, have it marked so I don't count it again and have a counter keep track.  Locator comes close but I would only be able to count one colour at a time.  Is there a way I can do this in R?

Thanks,
Doug