Rot-es is used here because it is nominative case, licht is neuter and, because there is no article, strong inflection is applied when declining the adjective. This link is very helpful... http://en.m.wikipedia.org/wiki/German_adjectives#Weak_and_strong_inflection
no determiner, nominative, neuter. m/n/f/pl: er/es/e/e. https://www.lsa.umich.edu/german/hmr/Grammatik/Adjektive/Adjektivendungen.html
Weak inflection after the definite article.