Hi there!
I just hit level 26 with the Valdrakken Accord and unlocked the drake armor but there doesn’t seem to be an option to change the color of the Grotto Netherwing Drake armor. In the customization videos on Youtube there’s an option but it seems to be missing now. Was that feature removed?
If the vendor does not have the [Highland Drake: Silver and Purple Armor] script, then it was likely removed. You do not buy one called Grotto Netherwing Drake.
I received one from the quest Armored and Ready from the Valdrakken Stable Master (https://www.wowhead.com/quest=70911/armored-and-ready) but it did not unlock any customization for the Grotto Netherwing Drake.
The weird thing is that it doesn’t even show a customization option for the armor. For all of the other drakes you can still see the armor colors despite not having them unlocked.
It’s possible it may have been a special item, or tied to something that has since been removed. Might be worth putting in a ticket and asking. WoWHead is limited in its details, but while there is a manuscript for the Grotto, one of the specifics points back to the Highland Drake making me wonder if the two are connected somehow.
Other than very rare drops from rares(not removed) and the pvp mount I dont think any dragon customization was removed…