you shout an arcane command, while tossing a handful of prismatic gem dust into the wind where it swirls into brief flashes of arcane symbols that quickly evaporate into flashes of polychromatic light. 

This spell is cast as a reaction to another spell of 5th level or less that is cast within range. The caster targets the creature casting the spell, and that creature has to make a saving throw against the casters spell save DC using their primary casting ability. (a wizard makes an Intelligence save, a druid makes a Wisdom save, a bard would save using Charisma, etc.) If the target fails their save you gain control of their spell and can select new targets or choices for the spell as if you had cast it yourself. The spells range, saving throw DC and other factors are now calculated using your abilities.

At Higher levels: When cast using a spell slot level of 6th or higher this spell can target another spell of the same level.