Re: Which color holds heat longest and why?

The answer to this one is a little surprising. White (or transparent) is the 
colour that holds heat longest! But it is not quite so simple: some sorts of 
white hold their heat better than others!

Heat is a form of energy. It is the energy associated with the random motions of 
the atoms or molecules of a material. There are three ways an object or region 
can lose heat. 
The first, conduction, is when an object loses some of its energy when fast 
moving molecules on its surface bump into slower moving objects on the surface of 
whatever is next to the object, and some energy is transferred. 
The second, convection (which really only applies to liquids and gases) is when 
the faster moving molecules move away to somewhere else in some sort of current, 
and are replaced by slower moving molecules.
In neither of these two cases does color as such affect the process. So we would 
have to compare objects that had the same conduction & convection properties. The 
best way would be by thinking about objects in a vacuum, where these two 
mechanisms cannot occur.

The third way of losing heat is radiation, which involves turning some of the 
heat energy into light energy. This is where color comes into the act. Energy can 
be lost by radiation even through a vacuum.

Now the wavelengths of light that an object can emit when it is hot are, by and 
large, the same as the wavelengths that it absorbs which determine its colour. 
White objects are white because they do not absorb visible light. So they cannot 
turn their heat into visible light either.

But unless objects are very hot -- red hot or hotter -- they do not turn their 
excess heat into visible light. Instead they emit invisible infrared light. For 
objects near room temperature the important wavelength range is about 5 to 20 
microns (millionths of a metre). Visible light wavelengths are 0.4 to 0.7 
microns.

Some white or transparent materials, like air and salt, are also transparent in 
the infrared region. Objects made of these materials retain their heat well. 
Other white or transparent materials, like glass or gelatin, absorb light fairly 
strongly throughout the infrared region. They do not retain their heat. Yet 
others, like water or polystyrene or polyethylene, have a few specific 
wavelengths of infrared that they absorb. They come somewhere in between.

It is even possible that some black or colored materials that absorb visible 
light strongly might be transparent in the infrared, and might retain their heat 
better than some white materials. I do not know of any, but it is not at all 
unlikely.