"The night is purple."
Translation:La nokto estas violkolora.
"Nokton" is only used when "night" is the driect object of a sentence. In this case, there is no direct object. A direct object would be like "ŝi legas libron". Its happening to a book, not just describing how the book is. "Nokton" would be for something like "a scream pierced the night", which i don't know how to say in Esperanto.