Fucking easy

DWORD rainbow( ) {

	static float x = 0, y = 0;
	static float r = 0, g = 0, b = 0;

	if( y >= 0.0f && y < 255.0f ) {
		r = 255.0f;
		g = 0.0f;
		b = x;
	}
	else if( y >= 255.0f && y < 510.0f ) {
		r = 255.0f - x;
		g = 0.0f;
		b = 255.0f;
	}
	else if( y >= 510.0f && y < 765.0f ) {
		r = 0.0f;
		g = x;
		b = 255.0f;
	}
	else if( y >= 765.0f && y < 1020.0f ) {
		r = 0.0f;
		g = 255.0f;
		b = 255.0f - x;
	}
	else if( y >= 1020.0f && y < 1275.0f ) {
		r = x;
		g = 255.0f;
		b = 0.0f;
	}
	else if( y >= 1275.0f && y < 1530.0f ) {
		r = 255.0f;
		g = 255.0f - x;
		b = 0.0f;
	}
    else if( y >= 1530.0f && y < 1785.0f ) {
        r = 255.0f;
		g = 0.0f;
		b = x;
    }

	x+=0.25f; //increase this value to switch colors faster
	if( x >= 255.0f )
		x = 0.0f;

	y+=0.25f; //increase this value to switch colors faster
	if( y > 1530.0f )
		y = 0.0f;


	return COLORCODE((int)r, (int)g, (int)b, 255);
}