@whenturtlesfly : It's "grünen" for the following reasons: no article is used (grünen Reis) (= strong inflection) + accusative case + rice is masculine. For the inflection of adjectives in German look at the information here: https://en.wikipedia.org/wiki/German_adjectives#Weak_and_strong_inflection
Couldn't it be because it's plural? Rice, not 1 grain of rice? Die Reis.
No, LearnerSojourner, "rice" is not plural--it is an uncountable noun (aka "mass noun"), and thus doesn't use a plural form (at least not in the common usage as a food item).
"Reis" is used similarly. "Reise" and "rices" are the plural forms. If one were speaking of "green rices" one would say "grüne Reise". That usage concerns varieties of rice and isn't applicable here.
Unripe fruits and vegetables are often called "green", even if they aren't that color when unripe, or remain that color when fully ripened.
But I've never heard anyone refer to raw rice as "green". I believe this is simply a reference to either a St. Patrick's Day novelty, or rice flavored with herbs.