Jeans are usually blue. They can come in any color. The material is called denim. Denim jacket is most commonly used, but it can also be called a jean jacket.
We (Americans, anyway) use "jeans" to mean any pants made of denim. If you didn't specify a color, I'd assume jeans are blue. But you can have white jeans or black jeans, and I used to have a fun pair of bright pink, leopard-print jeans. (See my comment in another thread about the 80's being a weird time.)
You can have a jean jacket. For some reason, when it's a jacket, it's "jean" in the