Probably needs some chelated iron.

Hard to tell from the picture so I would check for edema and also do a soil test to check for mineral deficiencies/pH problems.

I agree with Luis that soil testing is a good idea. If the problem is iron chlorosis or some other deficiency, a soil test should help you diagnose that. I'm not sure where in SoCal you are, but the link below from UC Master Gardeners of Orange County may be helpful.

You may also want to contact the Cooperative Extension Agency for your area- they may also do soil testing, and/or have some good info regarding local conditions that may be useful. Also, the American Camellia Society has a lot of info at their web site.

It's interesting that only one of your camellias is having this problem, though... Is it possible that that plant might have a drainage problem?

You may want to Google "camellia yellow mottle virus" for some images, but the leaves in your photo don't look like that to me.

Good luck!

Here is a link that might be useful: How to have your soil tested

Thank you all for the advice. I think I'll get a soil test. The link is very helpful.