"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.
The adjectives work like this:
indefinite, singular: et rødt æble (t-form because "æble" is a t-word)
definite, singular: det røde æble (e-form because definite marker "det)
indefinite, plural: røde æbler (e-form because plural)
definite, plural: de røde æbler (e-form because of definite marker)
possessive + indefinite, singular: mit røde æble (e-form with possessives)
possessive + indefinite, plural: mine røde æbler (e-form with possessives)