The colour and quality of the ink are fundamental to a stamp's appearance. Variations in ink composition, mixing, application or printing conditions can produce a wide range of colour varieties and ink-related printing flaws.

On stamps printed in very large quantities or over long periods, substantial shade differences may occur. Poor mixing or coarse pigment can produce a grainy impression. Excessive inking may tint areas that should remain light or white. Too much binder can create an oily impression that may also show through to the reverse of the paper. Thickened ink or excessive printing pressure can, in some processes, produce hollow or incomplete-looking impressions.

Ink show-through, offset impressions and printing on both sides should be distinguished carefully. In all three cases ink may be visible on the reverse of the stamp, but the processes that create them are entirely different.

Some stamps are known in several clearly distinguishable colour shades or varieties. Catalogues may list these separately, especially on definitive issues printed in very large quantities over extended periods.

Accurate identification of colour varieties is often difficult. Some catalogues provide dedicated colour guides for this purpose. Identification from photographs is even less reliable because lighting, camera processing and monitor settings all affect the colours seen on screen. Stamp inks may also change over time, especially after prolonged exposure to strong light or chemicals.

The term “colour error” is sometimes used for varieties that, according to the way they were produced, actually belong to another category. An incorrect denomination, for example, may be more accurately classified as a typesetting error.