Is it just an abbreviation for だった? Any nuance or just laziness?
1Related? japanese.stackexchange.com/q/66495/9831– Chocolate ♦Jul 7, 2021 at 14:07
デコった is the past tense of デコる, a slangy way to say "to decorate."
Oooh, the dictionary put it really far in the list so I didn't see it, I should've guessed, thank you– SimonJul 7, 2021 at 7:08
I think it's shortened usage of "デコりやがった" or more clearly "～しやがった(の(は))", which is strong way of saying "した" / "したの(は)", in this case the complete sentence will be "(俺の)右腕をデコレーションしたのピンクだな!?".
1As far as grammar goes, there is no やがる in the original sentence.– narutoJul 7, 2021 at 9:06
@naruto I... didn't wrote any やがる in my answer? where did you find it?– Skye-ATJul 7, 2021 at 9:38
1Ah, you meant デコりやがった → やがる? sorry, I just added that for clarity. Sorry if it's confusing.– Skye-ATJul 7, 2021 at 9:40