If you ask 1,000 people, you'll get 2,000 answers. Just make sure that the contrast ratio between your background and text is high enough that everyone (including those who are colorblind) can read it. Dark blue text on a dark green background may look sexy to the designer, but many potential users will take 1/2 look at the site and google to find a readable site with the same information. (I prefer a light blue or a light orange for themes, but that's just me.)
really any color can be good for a website as long as it's done properly. You should elaborate on what you are wanting to do with your "Green" and "blue" I mean, like is it the background, the elements? Your questions is a little vague. The most important part when choosing colors for a particular audience, in your case "weight" loss, is what your audience would expect to see. you should research to see what other weight loss companies are doing to get some ideas.
Just don't go around wondering for answers, pick the one that appears best to you. your opinion matters more than the opinion of a thousand other minds.
Green or Blue, any other colors? Anyway, just make it user friendly and don't make our eyes use glasses.
Please, visit some other fat loss websites. From that, you will get some ideas. Then you will design site with your best.
It depends upon what website you are building. Theme of the site impacts a lot on the choice of its color and design.
Another good place to get some inspiration about what colors combinations to use is kuler.adobe.com .