Some pear varieties like Bartlett pears change colors when they're ripe, but many varieties don't. The easiest way to check if a pear is ripe is to "check the neck" which means pressing on the top of the pear near the stem. If it feels a tiny bit soft and gives a little you'll know it's ripe and ready to enjoy. If it feels hard you'll want to leave it on the counter 1-2 days more to let it ripen.