Upper is an ocellaris.
Lower is true perc.
While every onyx is a percula, not every percula is an onyx. Onyx is generally reserved for percula fish that have black completely between the middle two bars, and also most typically having a dorsal fins also black. The black boarders on the tail and pectoral fins will also becocme thicker.
A true perc, on the other hand, may have some areas of black, but not completely black as described above. To call the above an onyx is a little bit of a stretch of the truth. The problem I have seen is individuals may have a pair of very nice onyx fish, and sell all the babies as onyx. Onyx parents will throw some percentage of onyx, but not all will achieve the coloration that should be reserved. The result is someone who has purchased a fish promised that it will color-up as onyx, but looks like you have shown above. Thus, perpetuating the discrepency between onyx and true perc coloration differences.
My avatar shows a pair of (I think) nice onyx.
If one were to purchase an onyx, you can only trust what your eyes see. Buy a black fish if that's what you want. Don't get an orange fish and a promise, you may be disappointed.