Is it red ?
It is red.
Which of each is correct or would both options be correct ?
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It only takes a minute to sign up.Sign up to join this community
Both are correct, and depends on what you are asking about.
If asking for the color of the traffic light, you would use あかですか / あかです
While waiting the lights to change, looking at other cars and due to e.g. sun being low, making it difficult to see the color of the car behind you in the rear mirror, it would be あかいですか / あかいです
"Normally" you would use あか when referring to the color itself, and あかい when using it to describe something.
(It is not only about colors, だい and おおきい are similar.)
But the above "normally" may be an overstatement, also considering the traffic light example. This is a reverse-engineered rule, but one tends to omit the い in combinations of noun and the adjective, when combined, are considered being one entity, being commonly used. For example, therefore, not only あかしんごう but also eg あかペン くろぺん だいじょっき.
(Without the last example, one could think that the あか in あかぺん and あかしんごう was there as one is not describing the color of the object, but the color it emits, but it does not explain だいじょっき.)