I would love to know the answer to this also. "Æble" is a neuter gender noun and from the grammar explanation it seems that "rød" should be changed to "rødt" when it is describing a neuter gender noun. I must be missing something, but what?
"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."
So it would be et rødt æble in the indefinite case but it becomes det røde æble for the definite case, because the definite case must be declined the same way as the plural case. And the plural case always seems to take -e, whether it's common or neuter.
When definiteness is marked by a pronoun (in this case the demonstrative pronoun "det"), the suffix is omitted from the noun.