Interesting color

If the material is has a single band (band gap Eg), the color depends on that only.

Eg	Tranmission/ color	reflection/ color	color of powder
>3.2 eV	large transmission / no color	small reflection / no color	white!
1.6eV< <3.2 eV	depends on thickness	depends on thickness	change color with powder size
<1.6 eV	none	metallic lusters may have different color (e.g. copper: red)	never change color with size

The color is very interesting and confusing. Actually, all the color we see is the color of transmittance color!!

Some may say, there are a lot of non-transparent colorful stuff. That's illusion of color. Let's use paint as an example. The paint contains many small transparent small particles (which is also called pigments). It is the reflection between interfaces makes the light not able to go all the way through, and in turn make the paint not transparent. The real color we normally see is the double transmitted light from the back surface.