"We eat the bright rice."
Translation:Vi spiser de lyse ris.
'den lyse ris' should be accepted. If you say 'de lyse ris' (plural) you would be talking about the grains, which are many, but if you use singular 'den lyse ris' you would be talking about the food called rice. You would probably mention the color because there is an option of having brown rice as well. (I'm Danish!)
When a noun has an attributive adjective in front of it, you would then use a definite article in front of the adjective and (this part is different from Norwegian and Swedish) use the indefinite form for the noun itself.
With no adjective, the plural definite form of “ris” is “risene” (no r), by the way.
Take a look at the "Tips" section of the lesson, it explains it very well : "While nouns normally express definiteness using a postfix, this changes to using an article if any adjectives (such as a color) is attached to the noun. If the color (or in general adjective) is used with a definite noun, then it is put between the definite article and the noun: En rød bil (a red car) becomes den røde bil (the red car). In this case the adjective is declined the same way as for the plural, no matter the grammatical number or gender of the noun."