Margaret, the phrase is ‘d’un vert printanier’, in the singular. Vert is a noun (masculine) in this expression, modified by the adjective printanier. If it was simply  “les arches vertes “, agreement would be required (feminine, plural).

Also for information, adjectival compound colour names with a noun modifier would be invariable.  

(It helps if you can include a direct link to the exercise or specific example with questions, as often context is important)

 Compound colour names and colour names derived from things are invariable (French Colour Adjectives)

 https://www.wordreference.com/fren/vert