Cat is throwing up saliva?

The yellow stuff sounds to me like bile, if it just happened for a short period of time it could be something simple like eating grass, but from the length of time I would suggest otherwise. The cat may have an infection or illness, it is definitely worth a vets trip. Also, you need to make sure she can access water as she will get dehydrated very quickly if she is throwing up a lot. I strongly recommend a vets trip as soon as possible.