wait is 赤い actually an adjective or what

so i was looking at this poster for like a gaming match (対戦?) and it said something about a red light like 赤い照明 but then i swear i've seen just 赤 on its own for the color red?? wait is this a dumb question lol why does it have the い at the end sometimes but not other times? like is 赤い a completely different word or did i just hallucinate the whole thing because my brain is melting trying to learn this