also: べたぬり
noun
solid painting; painting all over; covering in paint
Refers to painting a surface completely with a single color, often without gradation or leaving unpainted areas. Common in art, design, and DIY contexts.
背景をベタ塗りしてから細かい部分を描き始めた。
After painting the background solid, I started drawing the details.
このイラストはベタ塗りだけで仕上げてある。
This illustration is done entirely with flat coloring.
Compound of ベタ (from べた, meaning 'solid' or 'flat') and 塗り (the noun form of 塗る, 'to paint'). The exact origin is uncertain, but it likely arose in art and design communities to describe a flat, uniform application of paint.