It generally looks greener when there is alge and other organic matter in the water. On the Northern California coast it can be quite green at times and never that pure clear blue you see in other places. Visibility can be quite poor, sometimes just several feet. As you get further from shore there is less of this organic matter and the ocean takes on a bluer aspect.
In tropical waters there is often very little suspended in the water so it is clear and takes the natural blue of pure water. Here is an example of the water in Northern California