On Final Cut Pro X, is there a way to key out different shades of green rather than ALL things green?

I'm working on a project and have been using green screen, but found there were little bits of green (a different shade to the backdrop) on my shirt. when I use the Keyer, it keys out not just the backdrop, but those bits of green on my shirt too. Is there a way to make the keyer simply key out 1 shade of green rather than everything green?