"A red shirt is not blue."
Translation:Ein rotes Hemd ist nicht blau.
in other conversation is this answer http://en.wikipedia.org/wiki/German_adjectives#Weak_and_strong_inflection
This link showed me an easy way to remember the endings for all adjectives in German. Even has a 4-step flowchart to help out. Take ten minutes to read this, and I think it will help you too. :)
It's just a grammatical rule that when an adjective is used with the verb sein, nicht comes before the adjective. More on negation rules here: http://german.about.com/od/grammar/a/German-Negation.htm