Barlett pears change from green to yellow as they ripen.
Most others don't change color so much.
Because pears ripen from the inside out, the best way to check for ripeness is to (check the neck for ripeness). To do this, gently press near the stem with your thumb. When it gives to gentle pressure it is ripe, juicy and ready to eat. If you wait until the pear is soft around the middle chances are it will be overripe.