If the Knight has two colored mana symbols in its mana cost, you have one mana of either color available.
If the Knight has three colored mana symbols in its mana cost, you have one mana of any of those colors, and two mana of any other one of those colors available.
Steps
Activate Lotus Ring's equip ability by paying ({0} magic symbol) , attaching it to the Knight.
Activate the Knight by tapping and sacrificing it, adding three mana of any color.
Cast the Knight from your graveyard by paying its mana cost.
Repeat.
If the Knight has mana value 2 or less, this combo nets infinite mana.