"The toothpaste is white and green."
Translation:Tandpastaen er hvid og grøn.
When the adjective is attributive (ie. directly in front of the noun), you add -e if the noun is definite. Although this declension looks like the one for plural adjectives, calling the definite adjectives plural isn't really correct.
If the adjective is in the predicate, you simply use the indefinite declension.
"Den gode tandpasta er grøn."
"Den grønne tandpasta er god."