[The version with the 'e' at the end is used for both common and neuter plural nouns and for definite singular nouns when the adjective is sandwiched between a definite article and the noun. grøn is used with singular common nouns (which use the indefinite article "en" or as a Predicate Adjective for subjects which are common nouns) and grønt is used for singular neuter nouns (which use the indefinite article "et"). This is explained in the Tips & Notes:
"Adjectives decline according to the grammatical gender and number of the noun that they modify. In Danish you would say a green apple with the sentence et grønt æble. You have already seen et æble, so let us discuss the middle part: grønt. Grøn is the Danish word for green. When used to describe a neuter gender noun such as æblet the adjective is suffixed with -t and becomes grønt.
There are three ways an adjective can be declined: -, -t, or -e. - is used for common gender nouns, -t is used for neuter gender nouns, and -e is used for plural in both genders. This is also described in the following table“ (which is for endings added to the adjectives for use with nouns with indefinite articles):
” Common, Neuter
Singular - , -t
Plural -e , -e "
Okay the table did not copy well, so go back to the Tips & Notes for the better one:
Common singular nouns use the indefinite article "en"
Plurals can be reviewed here: https://www.duolingo.com/skill/da/Plurals]
But it gets tricky, because when an adjective is used with a noun, it no longer uses the postfix -en or -et . The plural version of the adjective is used and it is preceded by a definite article.
Here is where you may have been confused.
"en bil" "a car", "bilen" "the car", "en grøn bil" "a green car" so far so good, but "den grønne bil" "the green car"
From the Tip & Notes:
"Adjectives and Definite Nouns
While nouns normally express definiteness using a postfix, this changes to using an article if any adjectives (such as a color) is attached to the noun.
If the color (or in general adjective) is used with a definite noun, then it is put between the definite article and the noun: En rød bil (a red car) becomes den røde bil (the red car). In this case the adjective is declined the same way as for the plural, no matter the grammatical number or gender of the noun. As a reminder, the car without any adjectives is simply bilen, expressing the definite with the -en postfix and no article involved."
The plural would be "biler" "cars" and the definite plural without adjective would be "bilerne" "the cars"
Now if you were to say "The car is green.", the adjective is on the other side of the verb from the noun and uses the original form "Bilen er grøn." Here the adjective is used as a Predicate Adjective which refers back to the subject.