"The colors of the flag are red, white and blue."
Translation:De kleuren van de vlag zijn rood, wit en blauw.
Witte is for de words or when there is a sentence that has an indefinite or definite article and a de word , het words only get the extra "e" when there is a definite article before an adjective , "Het witte boek " "De witte stad " "Een wit boek " "Een witte stad " "De stad is wit " "Het boek is wit " I know im bad at explaning but have a read through this: https://forum.duolingo.com/comment/3888221/Grammar-Adjectives
It's because "des" is an old genitive form (similar to "des" in German too) that is used in some fixed expressions (e.g. de heer des huizes = the master of the house). We had an expression like this in the English course (Zij is vrouw des huizes?) for which we needed that hint, and it transferred over to the new course. I guess we can remove it now though, since we don't have a single instance of des in the Dutch course :P