Most NPC’s in Orgrimmar belong to the Orgrimmar faction, since you are Neutral with Orgrimmar it seems to me that is why they are yellow.
If you raise your faction above neutral their color will also change.
You can tell who an NPC is with when hover your cursor over them.
Here is someone affiliated with Orgrimmar when your reputation is at Neutral:
It will change when you raise it above Neutral:
There are other factions in the area too, such as Thunderbluff:
