"The color of the peach is orange."

Translation: Culoarea piersicii este portocalie.

[crbratu]

”Culoarea piersicii este portocaliul” would be considered acceptable by many Romanians. It is debatable if here ”portocaliu” is an adjective or a noun.

[Mapka15]

My answer:"Culoarea piersicei e portocaliu" was marked correct, but it is wrong, I suppose...?

[crbratu]

Not necessarily.
If you regard "portocaliu" as an adjective to the noun "culoare" then you should make the "acord" (Culoare portocalie).
But you could also consider "portocaliu" as a noun ( the name of the colour), and in this case it is not needed to make the "acord" ( Culoarea portocaliu)

[Jill635577]

Why is it piersicii instead of piersiciei? Thank-you.

[crbratu]

This is a common mistake in Romanian. An easy rule of thumb to build dative case for feminine nouns in romanian is to add "i" to the PLURAL nominative form. So piersică (sg) - piersici (pl) -> piersicii