Rot is the adjective, but sometimes it will be rote. When you say X is y and y is some kind of an adjective, then you always use the basic form. However, when it is directly with the noun, you have to put it in the corresponding case, so it would be Die rote Jacke, but Die Jacke ist rot.
This seems wrong. https://german.stackexchange.com/questions/15479/difference-between-jacke-und-mantel The coat is red. => Das Fell ist rot. Der Mantel ist rot. The jacket is red. => Die Jacke ist rot.