The background of Girl with a Pearl Earring appears black.
The nearly featureless dark field isolates the girl’s face, head covering, and earring, making the illuminated figure seem to emerge from darkness. This strong contrast is one reason the painting feels so intimate: there is no room, landscape, or furniture to distract from her glance over her shoulder.
There is an important conservation detail behind the apparently simple answer. Research found that the background was originally covered with a deep green glaze over a dark ground. The organic pigments in that glaze, including indigo and weld, faded over time. What viewers see today is therefore the result of aging and loss of color, with the background reading as black or almost black.
This does not make “black” an unfair quiz answer. It is the painting’s visible present-day background and the standard description used in general references. The subtle green history is a neighboring fact about the artwork’s restoration and changing appearance, not a competing everyday answer to the question.