"Pe piki" is "at the time when..."
"Piki lun" is a noun phrase, not an adverbial phrase. It could be the subject or object of a sentence, but to get the meaning desired for this sentence, it needs to be the object of a preposition.
This might make sense, but since it's talking specifically about time, pe piki would probably be used instead.
...not sure how to translate the "to be their color" part, FIXME
TODO this sentence seems confusing