The leaves of my bonsai pomegranate tree, which is kept outside, are turning yellow. I live in the California desert, but the temperatures are very nice right now, sometimes dipping into the high 40s during the night, but the 70s during the day.

I just planted the tree, a few weeks ago, in a shallow bonsai pot with the help of a bonsai specialist. Do I need to worry about the yellow leaves? I won't be able to see the bonsai master who helps me for another couple of weeks.

Sounds like the result of re potting and not a problem unless there are a lot of leaves dropping. Just keep to the proper watering, when needed not to a schedule, and it should be fine. I expect you were told to keep it in a shady spot for a few weeks, if not then that is what I would do.
I have one allbeit in a deeper pot which has been outside for the past two winters and we have had temperatures as low as -12C It does take a while to get going again, though.

Thank you for your help, I appreciate it very much. Leaves aren't dropping and it's been in partial shade, but I'll move it so it gets more shade.

Hiya. With my pomegranates, this is the time of year many of their leaves naturally turn yellow and start to fall off, particularly if it goes down to certain temperatures.