It is clearly noticeable in the second picture you can see the back of it the velcro.And who ever didn't notice obviously Dosent know what a plum headed parakeet looks like because they don't have blue feathers.

Plumheads are rather uncommon, so not too many people know the details of exactly what they look like.

Not to mention, not everyone is incredibly active and checks unread posts daily here, so it's a possibility that they didn't see what Mango looked like without the harness on.