I translated this as 'It is a pretty light gray' and was marked correct. But that made me question whether 'pretty' in this context meant 'lovely' or was an adverb reinforcing 'light'.
So on reflection, I think my answer should not be accepted, because in english we would commonly use 'pretty light' in the sense of pale. To convey the intended meaning, I would instead say 'This light gray is pretty.'
Your statements appear contradictory to me. I understand your BANGS rule to imply that the color light/pale gray is lovely/pretty.
But the American English "pretty light gray" would mean that the color gray is very/extra pale --, i.e.., 'pretty light'.
Hence my statement that an english translation that conveys the French meaning would either not use pretty as the modifier (i.e., lovely), or move pretty to the end.
Wmconlon, you are absolutely correct that the word "pretty" is commonly, emphatically and colorfully used, at least in American English, to indicate "very/extremely/extra/fairly/really,really/quite". But it is also "pretty" close to slang, and now (quite) dated.
I think that most people will have no trouble understanding, correctly, your initial translation.
It was, however, an interesting consideration.
Certain adjectives precede the noun - known as the BAGS or BANGS adjectives:
Beauty ............ joli/jolie, beau/belle
Age .................. jeune, vieux/vieille, nouveau/nouvelle
Number .......... un(e), deux, trois, seconde, deuxième, etc
Goodness ....... mauvais/mauvaise, bon/bonne, gentil/gentille
Size .................. petit/ petite, grand/grande, gros/grosse, long/longue, énorme
Aha! The expression "pretty light" to mean "very light/pale" would not be preferred. One would probably say "very light/pale" or "rather light/pale". To use "pretty" to modify "light" is to use pretty as an adverb. But "joli" is not an adverb and cannot be used that way.