Actually, you don't even need Gouraud, since the Self-Illumination
being 100 will prevent any Ambient shading from appearing anyway
(thus no facets will be apparent). Use Flat, since it's faster.
Also, you should make sure that both Shininess and Shininess Strength
are set to 0. The Specular color thus becomes irrelevant (just as
Self-Illumination makes Ambient irrelevant) and only the Diffuse
color matters. With Self-Illumination at 100 and Shininess/Sh. Str.
at 0, every pixel of the faces with this material (except at
anti-aliased edges) will be precisely the Diffuse color, even with
Flat shading, regardless of illumination, shadows, etc. (Atmosphere
could affect the color, though).