Trade Port

{w}{t}, add {u} to your mana pool.
{u}{t}, add {b} to your mana pool.
{b}{t}, add {r} to your mana pool.
{r}{t}, add {g} to you mana pool.
{g}{t}, add {w} to you mana pool.
Whenever you cast a spell using mana generated by trade port, untap trade port.