"I eat a yellow apple."
Translation:Mi manĝas pomon flavan.
I believe "pomon flavan" means the same as "flavan pomon". It doesn't matter which way round the adjective and the subject go; e.g. "Mi havas multajn hundojn" is the same as "Mi havas hundojn multajn." To be honest, you should probably wait until a fluent someone, like salivanto sees this comment.
From the notes for Weather:
"In Esperanto, the adjective may be placed either before or after the noun, with no change in meaning. "Bona tago" and "tago bona" both mean "a good day" and both are correct. In practice, most people prefer "bona tago", with the adjective before the noun."
You need agreement between the adjective and the noun. Since the noun takes the -n (because it is the direct object), then the adjective needs the -n as well.
This works for plurals as well. If it were "I eat yellow apples", then the translation would be "Mi manĝas pomojn flavajn" or "Mi manĝas flavajn pomojn."