Why not the plural form for "gray"?
"gris" is the same for singular and plural masculine nouns.
Exactly my question. Why is it not "costumes grises"?
'We order some gray suits.' isn't accepted.
Neither is "Let's order grey suits," despite the fact that the drop-down menu suggests "let's order" for "commandons."
If you said "Commandons des costumes gris" (i.e. use the imperative form) then it does mean "let's order". If you say "nous commandons" it means "we order".