can Ostiary Thrull ability tap creature with protection from white?

[Swamp]

since the ability of Ostiary Thrull requires white mana to cast, can it target pro white creatures?

[Arix]

Yes. The cost of the ability doesn't matter, what matters is the colour of this source - in this case, black. It can tap something with prowhite, but not problack.

[hideseek]

but when you cast a spell doesnt the color of the spell is determined from colors of mana in cost? so when you cast an ability isnt it the same?

[Void Elemental]

With abilities, you look at the color of the source.