To color a flat colors 3D model, use 3D texturing software (like Blender, Substance Painter) to UV unwrap the model, then apply solid colors to each UV island or face.
Flat coloring relies on uniform, solid hues (no gradients or textures). After unwrapping (mapping 3D surfaces to a 2D canvas), you can paint directly on faces or assign solid colors via material settings—common for stylized games, animations, or minimal designs.
For beginners, try Blender’s "Texture Paint" mode: Use "Smart UV Project" for quick unwrapping, select a solid color, and fill faces with a brush (no complex texturing needed).