...
uniform sampler2D extra_texture;
in vec2 texture_coords;
out vec3 final_color;
...
void main() {
    ...
    // Assume color1 to be the base color for the sphere, no matter how you get it (attribute/texture), it has at least 3 components.
    vec4 color2 = texture(extra_texture, texture_coords);
    final_color = mix(vec3(color2), vec3(color1), color2.a);
}