Grammatically, you can never use an adjective with の. You can only use の with nouns. You're confusion is arising because the parts of speech do not always correspond between English and Japanese: 次 is a noun, but "next" is an adjective.
Similarly, all colors in English are adjectives, but all colors in Japanese are nouns (except for six that have adjective counterparts, namely 赤い, 青い, 白い, 黒い, 黄色い, and 茶色い). You can't say 灰色い because it only exists as a noun, not an adjective (a fact you just have to memorize).
You can use nouns to modify other nouns in English, too (and probably most other languages). Health Department (or Department of Health) is an example in English: "health" is modifying "department" to describe what kind of department.