The color differences are from different "tone mapping" implementations after the render is stopped and can be ignored.

The point of that page is to evaluate renderer correctness (think of them as visual unit tests), not to really compare "which one looks better". For example, some of the images show appleseed with hard shadows, when they should be soft.

If you want to know which one is "right", Mitsuba is the one to look at as it's generally the most correct.

The graininess comes from not letting the machine renderer longer—the longer it goes, the less grainy it will be. That said, all of these renderers employ "tricks" to reduce the graininess that can make it practically impossible to remove whatever graininess slips through.