You can color a flat-color 3D model without Blender using simple tools like Tinkercad, SketchUp Free, or online editors (e.g., Pixlr, Photopea) for solid hue application.
Flat coloring uses solid, unshaded colors—these tools let you apply them directly to model faces or edit flat texture files (like PNGs). For example, Tinkercad’s "Color" tool assigns solid shades per shape, while SketchUp Free’s "Paint Bucket" adds flat materials.
If you’re new, start with Tinkercad—its drag-and-drop interface makes flat coloring easy without learning complex software.