"I have a light blue car."
Translation:Mám světle modré auto.
Agreed. I think “světle” is an adverb here, and “světlé” is an adjective. I think that, by this point in the course, we’ve probably come across this construction before (e.g., “dobrý” / “dobře”, etc.). Akin to the -ly adverb ending in English.
To my anglophone ear, “light blue” is a color, where “light” functions as an adverb describing “blue”. Describing a car as “light” would imply weight rather than color (e.g., “I like their line of light pickups”), so you would have to say, “I have a light-colored car” or something to that effect. But light blue is a light color, so we’re probably down to semantics here...
I think we have to accept it together with "Auto mám světle modré." which better shows the difference. The verb acts as a sort of copula here but because we cannot translate this into English directly I feel we have to accept it as a translation in this exercise. It more or less means My car is light blue.
It is not the same as placing the agreeing modifier after the noun *"Auto světle modré je na ulici." — that is wrong in normal conversation or prose.
Placing the adjective after the noun makes it sound poetic. This is something you might find in a song or a poem. The standard, regular word order is always adjective+noun, i.e. světle modré auto. "modré" is an adjective that specifies the noun "auto", while "světle" is an adverb that specifies the adjective "modré".