Tar is usually black or very dark brown because it contains carbon-rich compounds, heavy oils, and fine dark residues formed when organic material is heated and broken down.
The darkness of tar is one of the reasons the word is used so loosely. Many unrelated dark materials get called tar in everyday speech. True tars are dark because heat changes wood, coal, bark, peat, or other organic matter into a mixture of heavy liquid compounds and carbon-rich residues. Those substances absorb light instead of reflecting much of it back to the eye.
Fresh tar is not always a flat black. Some wood tars can look reddish brown, amber brown, dark honey brown, or glossy black depending on the feedstock and the way they were made. A high-quality Stockholm tar may look browner than a heavy coal-tar product, especially when spread thinly on wood.
Thickness also changes the colour. A thin film of tar on timber may show warm brown tones, while a deep pool in a tin can look nearly black. As tar ages, it can darken, dull, gather dust, or form a skin. The same material can therefore look different in a tin, on a brush, on rope, and on weathered timber.
Blackness alone does not identify tar. Bitumen, asphalt, soot, pitch, rubber, oil residues, and many modern sealants can all look tar-like. Colour is a clue, not a definition. The origin and behaviour of the material matter more.