Why is my snake breathing with mouth open and has yellow scales?

It sounds like she either is trying to regurgitate a meal or she has a respiratory infection. When was she last fed? How large was the prey? How often do you feed her? Do you hear any wheezing? Any bubbles coming out of the nose or mouth? I don't really have enough information here to be helpful.

The yellowing of the scales appears to be mild scale rot. You can try treating her at home with dilute betadine solution (1 part betadine to 10 parts water). If she has yellow scales all over her body you can soak her in a betadine solution bath for 20 minutes TWICE a day every day until the infection clears up. If it is just the location in the photo you can just apply the dilute betadine directly to those areas with a cotton swab. The betadine solution should be 10% betadine and the water should be lukewarm (80-85°F). After bathing, rinse the snake off and apply a topical antibiotic ointment (something like Neosporin will work fine).

If the infection doesn't get better or even worsens, discontinue the betadine baths and seek professional help immediately.

If you would like to post again with more information or opt for a private consult we may be able to help you better. Hope this helps. Best wishes.