Citrus don't ripen after they are removed from the tree; some things may change the color, but not the ripeness. Go ahead and cut it open and use it. In the future... if the fruit don't "jump off the tree", IMHO the Bearss lime is best when it begins to get a bit of yellow color. If you are a Brit, or such, and just love that horrid bitter flavor of "limes" for things like gin and tonic, you can pick them earlier. BTW the difference between "lime" and "lemon" depends in large part upon what language you are speaking; in English lime means green; and lemon means yellow; in Spanish they are all limon (lemon).