La fooire aux fleurs

Question

Then we cross the spring-green vine arches,Puis nous traversons les arches de vigne d'un verts printanier,Puis nous traversons les arches de vigne d'un vert printanier,Does the green not refer to the arches (Plural)?

Accepted Answer

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)
 
 https://progress.lawlessfrench.com/revision/grammar/compound-colour-names-and-colour-names-derived-from-things-are-invariable
 https://www.wordreference.com/fren/vert