"It is dark."
Translation:Tá sé dorcha.
Permanent/temporary attribute has no relevance to the use of the copula. You won't get a more permanent state than "dead", which is Tá sé marbh, and being a tourist is typically temporary, but it's still Is turasóir mé.
The difference is that marbh is an adjective (that describes a state that happens to be permanent), and turasóir is a noun.
"dark" is an adjective. You don't use the copula is when you are using an adjective to describe a pronoun (sé).
dorcha is a both a noun and an adjective, so it is grammatically possible to say Is dorcha é, but it doesn't mean "it is dark", it is closer to "it is a (period of) darkness".
Question about word order: I answered this one correctly but wondered if the word order changed slightly how it would affect the meaning. Tá dorcha sé. Is it saying the same thing but putting more emphasis on the fact that it's dark? I should be farther along in understanding this by now, but no one to practice with so...