Colour an attribute of things that results from the light they reflect, transmit, or emit in so far as this light causes a visual sensation that depends on its wavelengths. Colour derives from the spectrum of light   interacting in the eye with the spectral sensitivities of the light receptors. All light rays contain colour. Light is made of electromagnetic waves. These waves spread out from any light source, such as the sun. Colour attributes were first understood by 19th century physiologist Ewald Hering.  Colour categories and physical specifications of colour are also associated with objects or materials based on their physical properties.

Dyes

Dyes , colouring materials that are applied as a solution and cling to whatever they are applied to can be used for decorative, aesthetic, and artistic purposes. Fluorescent brightening dyes improve the appearance of textiles and paper. Dye based lasers are not only powerful, but they also emit laser light over a range of wavelengths, producing tuneable laser beams for manufacturing medical applications and astronomical research. Dye laser beams can even be used to vaporize tattoo inks. Dyes are intensely coloured or fluorescent organic substances only, which impart colour to a substrate by selective absorption of light.

Pigments

Pigments are coloured, black, white or fluorescent particulate organic or inorganic solids which usually are insoluble in, and essentially physically and chemically unaffected by, the vehicle or substrate in which they are incorporated. Pigments are often derived from minerals, but they can also be made synthetically. Pigments are used in paints, inks, plastics, fabrics, cosmetics, and food. Its insoluble materials that have to be mixed with binders or vehicles to attach them to a substrate. Inks are liquids or pastes that contain dyes or pigments.

Dyes and Pigments

Dyes are soluble, lower as compared to pigments during application process there is a temporary alteration in the structure of the dyes. Dyes can only impart colour by selective absorption of the dyes usually the dyes are organic compounds. The dye based printing inks do not last as long as the pigment inks. Compatible with almost all the substrates that needs to be dyed. Dye molecules are comparatively smaller it’s like comparing a football (pigment) to say a head of a pin (dye) .While pigments are normally inorganic compounds, often involving heavy toxic metals. In case of ink based printing prints made with pigments lasts longer. Owing to the physical makeup of the pigment inks the range for suitable substrates are limited. Traditionally pigments have been found to be more lightfast than dyes. During application, pigments have the capacity to retain particulate or crystalline structure Pigments impart colours by either scattering of light or by selective absorption. Pigment particles are about 1-2 microns in size.

Alizarin

Alizarin, (1,2-dihydroxyanthraquinone ) is an organic compound .it’s a red dye originally obtained from the root of the common madder plant. Historically it was derived from the roots of plants of the madder genus. Cloth dyed with madder root pigment was found in the tomb of the Pharaoh Tutankhamun. Madder has been cultivated as a dyestuff since antiquity in central Asia and Egypt, where it was grown as early as 1500 BC. Laboratory methods of preparing alizarin from anthra quinone were discovered in 1868, and, upon commercial introduction of the synthetic dye in 1871. Alizarin is one of ten di hydroxyl anthra quinone isomers.

Cadmium sulfide

Cadmium sulfide is a chemical compound that has the formula CdS. It is yellow in colour and is a semiconductor of electricity. It exists as two different polymorphs, hexagonal greenockite and cubic hawleyite. Cadmium Sulfide is also used in the production of solar cells where it is used as a buffer layer in the manufacture of CIGS (Copper -Indium-Gallium-Selenide) solar cells. With an increasing interest and uptake of solar cells, this application for cadmium sulfide could also increase. Screen printing using a slurry containing dispersed CdS and Light dependent resistors, photo resistors for light sensors.

Lead Chromate

Lead Chromate is a yellow, orange or red coloured, crystalline, inorganic compound that emits toxic chromium fumes upon heating. The solubility of lead chromate in water is exceedingly small; indeed, the chromate is probably the least soluble salt of lead.  Lead chromate is highly corrosive and is a strong oxidizing agent. This substance is used in printing inks, paints and to colour vinyl, rubber and paper. Lead chrome yellow turns to black with hydrogen sulphide. Lead chromate primarily affects the lungs causing shortness of breath, bronchitis, pneumonia and asthma but can also affect the gastrointestinal tract, liver, kidneys and immune system. This substance is a known human carcinogen and is associated with an increased risk of developing lung cancer and cancer of the sinonasal cavity.