Yes, you can still attach auras to it as long as they aren't being cast normally.
702.16m Some Auras both give the enchanted creature protection and say “this effect doesn’t remove” either that specific Aura or all Auras. This means that the specified Auras can legally enchant that creature and aren’t put into their owners’ graveyards as a state-based action. If the creature has other instances of protection from the same quality, those instances affect Auras as normal.
303.4f If an Aura is entering the battlefield under a player’s control by any means other than by resolving as an Aura spell, and the effect putting it onto the battlefield doesn’t specify the object or player the Aura will enchant, that player chooses what it will enchant as the Aura enters the battlefield. The player must choose a legal object or player according to the Aura’s enchant ability and any other applicable effects.
Also, although reminder text is not official rules text, the reminder text on Spectra Ward gives the answer:
(It can't be blocked, targeted, or dealt damage by anything that's white, blue, black, red, or green.)
Note that it leaves out the "or enchanted" part of the reminder text that would normally encompass what protection means.
The only reason you can't cast an aura normally to attach further auras to the enchanted creature is that auras are targeted spells.