What is a common reason if blended colour appears lighter than expected after drying?

Correct answer: Wet colour appearing darker than dry colour

The governing concept is the wet-to-dry colour shift. Water, binder, and surface reflection can make a freshly applied colour look richer or darker while it is wet; after drying, the appearance may become lighter or less saturated. Thus option D gives the normal explanation. Brush size does not change during drying, black would generally darken the mixture, and a spontaneous paper-colour change is not the usual cause.