stingc

No. A star's color has to do with its temperature. Any hot object gives off light. This light is spread over a wide range of wavelengths (colors), although it does have a peak. Stars can have a peak intensity in the green part of the spectrum, but the range of wavelengths given off is so wide that our eyes don't interpret that as green. It usually looks more like white or yellow.

A red star is not very hot, so its peak is usually in the (near-) infrared. There is still enough radiation in the red region of the spectrum that we'll see the star as red, but there isn't enough at higher frequencies for our eyes to get "confused." A similar thing happens with blue stars.

As for the pond, it is a combination of what Charlatan and others said. No matter how the water is cooled, the coldest portion will be the least dense, and will therefore rise to the top (this only works near the freezing point - the opposite effect happens in warm weather).

Think about the gas as a collection of very quickly moving molecules bouncing around. They are falling down (and flying up) all the time. Gravity does affect this, which is (one reason) that the air pressure decreases with increasing altitude. The molecules lose speed as they fly upwards and gain speed when falling down. The macroscopic description is rather different, so I won't try to confuse you.

edit: C4 beat me to it Gas molecules don't hit each other very often though.

Also, energy levels have nothing to do with this. Thermal radiation is a separate process. Energy levels only give miniscule corrections to an overall spectrum that is nearly continuous.