why cadmium is yellow?

Cadmium pigments are a class of pigments that have cadmium as one of the chemical components. Most of cadmium produced worldwide is used in the production of Ni-Cd Batteries, but about half the remaining consumption, which is about 2,000 tons annually, is used to produce colored cadmium pigments. The principal pigments are a family of yellow/orange/red cadmium sulfides and sulfoselenides. Cadmium yellow is cadmium sulfide (CdS); by adding increasing amounts of selenium, colors ranging from orange to nearly black (the color of cadmium selenide) can be produced. Cadmium yellow is sometimes mixed with viridian to give a bright, pale green mixture called cadmium green.Brilliantly colored, with good permanence and tinting power, Cadmium Yellow, Cadmium Orange, and Cadmium Red are familiar artist colors, but of little use in architectural paints. Their greatest use is in the coloring of plastics and specialty paints which must resist processing or service temperatures up to 300°C. The color-fastness or permanence of cadmium requires protection from a tendency to slowly form carbonate salts with exposure to air. Most paint vehicles accomplish this, but cadmium colors will fade in fresco or mural painting. Cadmium pigments can also color glass and ceramic glazes, not by solution, but colloidal dispersion within the glass. The lenses of red stoplights use this technique.[citation needed]

Cadmium sulfide and a mixture of cadmium sulfide with cadmium selenide are commonly used as pigments in artists' paints. They have an excellent reputation for color permanence although this is partially based on two reasons which are not necessarily directly related to their properties:

1. when introduced, there were hardly any stable pigments in the yellow to red range, especially orange and bright red was very troublesome, when the cadmium pigments replaced e.g. mercury sulfide (the original vermilion), the light-fastness was greatly improved,
2. companies sell the cadmium-containing paints at premium price. Although the pigments are certainly more expensive, the premium price is often not fully justifiable, with reasons more in the marketing area than in the actual raw material cost.

Nowadays, the cadmium pigments have been partially replaced by azo pigments. These are similar in lightfastness to the cadmium colors and have the advantage of both being cheaper and non-toxic. With respect to lightfastness the lemon yellow cadmium pigment is an exception: the azo-variety is highly superior in light-fastness.[dubious – discuss] In some countries, such as Australia, consumer activists such as Michael Vernon were successful in banning the use of cadmium pigments in plastics that could be used for toy manufacture, owing to the toxicity of cadmium.

http://en.wikipedia.org/wiki/Cadmium_pigments