For me a good place to start: what is light?
Light is some kind of electromagnetic field that has associated property known as wavelength. The shorter the wavelength the more energy it carries. Conversely the longer the wavelength the less energy is carried so the intensity of light in a given wavelength might be different than the intensity of light at another wavelength. How light intensity changes with wavelength is the lightís spectrum.
We perceive lights different wavelengths as colors. The longest wavelengths are red and as the wavelength decreases (gets shorter) we see all the colors of the rainbow to violet, the shortest light wavelength we see.
Another way to talk about lightís wavelength and the associated color is the Kelvin scale. Kelvin ratings (degrees) measure the color temperature (spectrum) of light. Color temperature is not the heat of the source of light it is the color given off by the source light.
Why is the hue (color) of the light measured as a temperature?
Back in the late 19 th century a physicist named Kelvin heated a block of carbon. The carbon glowed in the heat giving off different colors at different temperatures. The carbon at first produced a red light that changed to yellow as the temperature increased and eventually produced a white-blue light at the highest temperature.
So the longest light wavelength with the least energy (red) has the lower k rating and lower color temperature than shorter light wavelength (blue) with more energy has higher k rating and higher color temperature.
In "light" of this information it should be cool red and hot blue.
When light travels through water it is absorbed. The longer wavelength are lost first, so the shorter the wavelength the deeper they penetrate the water column. The descending order of penetration by color is red, orange, yellow, green, blue, indigo then violet.
Why use a full spectrum bulb in the aquarium if most of the light wavelength is lost in the first few inches of the water column: is it because of algae?