Licht is a neuter noun and there is no preceding article = strong declension: http://en.wikipedia.org/wiki/German_declension#Strong_inflection.5B6.5D.5B7.5D
The ending changes depending on wether it is before a die, der or das word. Usually you say DAS licht but because it misses out das, you change the end of rot (the adjective) to rotES. rotEN rock. (der) rotE jacke. (die) rotES licht. (das) Works like this for all the colour words and words likes groß and klein I think.