If you have alkaline soil (or hard water) then it might be chlorosis. Try looking at google images for "camellia chlorosis". If that is the problem, there is special fertilizer for acid loving plants.

If the camellia was planted quite recently, it may just be transplant stress. (Fertilizing a stressed camellia is not recommended, but if you used an organic plant food such as Holly Tone, that should be fine.)

Another possibility might be if the shrub was planted too deeply. Camellias have shallow root systems, and can't "breathe" properly if planted too deeply.

I am assuming you've already checked to be sure the planting area has good drainage.

You may just need to wait to see if new foliage is also yellow to know if you have a problem that needs to be diagnosed/treated.

Good luck,

Virginia