Rot or Rote?
What is the difference between "Die Jacke is rot" and "Die Jacke ist rote"?
Predicate adjectives are never inflected (Die Jacke ist rot). http://en.wikibooks.org/wiki/German/Grammar/Adjectives_and_Adverbs
To add to Jena's comment and hopefully provide more explanation... When the adjective is linked to the noun through a verb, in this case "rot" is connected to "Jacke" through "ist", then the adj does not take on an ending (-e, -es, -en, etc depending on the case Nom, Akk., Dativ, Genitiv and the gender of the noun). Other examples can be- "Das Haus ist blau", "Der Mann ist alt." etc
When this is not the situation, then the adjective takes on an ending required by the case and gender of the noun. Examples for the nominativ case: "Die rote Jacke...", "Das blaue Haus...", "Der alte Mann...", "ein alter Mann..."