Tar can change colour in sunlight because ultraviolet light and air alter the outer layer. Fresh glossy black or brown tar may slowly become duller, greyer, browner, or more uneven.
Sunlight is one of the main reasons tar does not look the same forever. Ultraviolet light affects the surface chemistry, while heat from the sun changes softness and flow. The topmost layer gradually becomes different from the darker, softer material underneath.
On wood tar and pine tar coatings, sunlight can turn a rich brown-black finish into a flatter black, grey-black, or smoky brown surface. Dust and pollen can also cling to the tacky surface early in the coating life, changing the colour before the tar has fully settled.
The colour change is strongest where sunlight, rain, and temperature swings all work together. A sheltered underside may stay darker and glossier, while an exposed face becomes dull and weathered. This is why old tarred buildings and fences can look patchy even when the same material was applied.
Colour change does not automatically mean the protection has failed. A tarred surface can be faded and still useful. The signs that matter more are cracking, peeling, bare wood, persistent stickiness, trapped moisture, and areas where water no longer sheds properly.