Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry that Luna is not feeling well...I know this is scary for you and for her.
In most cases, this is a mild digestive disturbance caused by something odd she may have ingested and this yellow color is due to bile being produced after all teh food is gone. The best treatment for this is as follows:
Fast Luna for 12-24 hrs to allow the digestive tract to calm down and heal. I am glad that you are already doing this.
Give Luna Pepcid AC or Famotidine at 0.5 mg per pound every 8 hrs. That can be purchased over the counter at the pharmacy.
Give only small amounts of water during the fast or ice cubes. It is good to give Pedialyte, 2-4 ounces every 4-6 hrs to be sure she is not dehydrated.
After the fast, feed rice and boiled chicken (2/1)
If the vomiting has resolved, then wean Luna back onto regular food. If the vomiting persists especially during the fast, take Luna to your vet for an exam and lab work to determine the cause for this. This is extremely important since this can cause dehydration or other more serious concerns such as an obstruction. The good news is that, in most cases, this will resolve this for Luna.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, ***** *****