Normally, yellowish-white vaginal discharge is composed of cervical mucus, discarded cells and harmless bacteria that are normally present in the vagina. Yellowish-White discharge is considered normal if it remains yellowish-white and is only a small amount. If there is change in color, amount, or if you begin to notice an odor, a vet visit would be needed to ensure that there isn't a problem.
A green-yellow discharge means infection and would need to be treated by a vet. This could potentially mean pyometra, an infection in the uterus which can be fatal if not treated. If your girl begins acting lethargic, stops eating, or in any other way, seems somehow just "not quite right", then you should get her in to see her vet.
Just in case, have you provided your girl with a safe place to deliver? If you haven't already, please take a moment to read this article
in the event you are expecting kittens.