A green square.

main.cpp

void drawStuff() {
    static GLfloat wtf = .0f; static bool way = true;
    glColor3f(.0f, (wtf+=.002f*((way=(wtf<.0f||wtf>=.6f)?!way:way)?1:-1)), .0f);
    glBegin(GL_QUADS); {
        glVertex2f(0, 0); glVertex2f(400, 0);
        glVertex2f(400, 400); glVertex2f(0, 400);
    } glEnd();
}