The choice of colorant depends not only on processing requirements, but more importantly on the final use of the product. The following characteristics must be taken into account when using it: resistance to fading (caused by sunlight, heat or wind and rain erosion) resistance to bleeding, and detrimental to the basic rubber aging characteristics (avoid using colorants containing copper or manganese).

In addition, it is necessary to calculate the cost and carefully select the most economical colorant to meet the production needs of colored rubber products.

There are two groups of colorants available: inorganic colorants and organic colorants.

1. Inorganic colorants

The inorganic colorants commonly used are iron oxides, lead colorants (such as chrome yellow), ultramarine, chromium oxide and cadmium colorants.

Iron oxide can be red, yellow, brown or brown. All of these shades are relatively dull and are all supplied in a fairly fine, high-density powder. Synthetic iron oxide colorants are recommended. Special care must be taken when selecting iron oxides because some iron oxides contain impurities (such as copper) that can cause rubber products to degrade when they age.

Ultramarine colorants provide shades ranging from green to reddish blue. These colorants have excellent heat and light stability, but poor acid resistance, which limits them to many outdoor applications and any indoor use where they may come into contact with acidic substances such as juice. Ultramarine's greatest use in rubber is in white rubber, just as indigo is used when washing shirts. Ultramarine produces saturated, bright colors.

Lead colorants include chrome yellow and molybdenum orange, which have a wide range of tones. These colorants produce bright colors with good heat and light resistance, but they are lead compounds, and their dust can glow and burn. In sulfur-cured rubber, any moisture may cause lead sulfide spots. Currently, these colorants have been replaced by organic colorants in most cases.

Cadmium colorants are only used in special applications where non-fading is required. This colorant is usually added in basic form to improve its dispersion and processing. It has a high relative density and is expensive. Cadmium is also a heavy metal, so rubber compounds should be avoided as much as possible.

All inorganic colorants are weak colorants, and their normal dosage is 2% to 5% of the total mass of the rubber. Inorganic colorants are generally easier to disperse than organic colorants, their particle size is larger, and their relative density is much higher, so there is less dust flying. But this is only relative to organic colorants. In fact, all colorants have serious dust flying, and preventive measures must be taken during the production process.