Vertex color
A colour stored per vertex of a mesh and interpolated across the triangle — one of the oldest ways to colour geometry without a texture.
Each vertex carries an RGBA value alongside its position, and the rasteriser interpolates it across the triangle, Gouraud-style. Before textures were cheap, vertex colours were how you shaded a model at all.
They never went away because they're nearly free and double as a general per-vertex data channel: artists paint them to blend terrain textures (splat weights), fake ambient occlusion, tint foliage or mask effects in a shader. The limitation is resolution — detail can only vary as finely as the mesh is tessellated, so a flat wall needs geometry it would otherwise do without.