The phrase "full opacity" means complete and total transparency, where nothing is hidden or unclear.
Sentences with «full opacity»
The base is more of a crelly type base that reached full opacity in just 2 coats. (cosmeticsanctuary.com)
The store states that you should apply 2 - 3 coats of this polish for full opacity however on my third coat I decided to add one extra just for good measure. (ithinitybeauty.com)