Every vertex has texture coordinates associated with it.
When a polygon is filled in, the vertices' texcoords are interpolated to get the texcoords of each pixel drawn.
If glTexCoord is not called for a vertex, it
inherits whatever texcoord was previously set.
The default coordinates, if glTexCoord is never called,
are (0,0).
If all vertices of a polygon get the same texcoords, then the polygon's pixels will all be filled with the same color from the texture.