Invoke fey colors that inflict random magical effects
Every magic spell has an aura comprised of colors that can be seen by those with the right senses. The color of magic speaks volumes about how the spell works, as each color corresponds to a different aspect of reality. Fey warlocks can produce pure colors that produce magical effects, inverting the normal relationship between color and magic.
Create a wall of color that can block certain effects and cause various helpful or hindering effects on creatures that pass through it
Notes
At low level, colors tend to be random. At mid level, you can choose a color. At higher levels, you can use multiple colors at a time. At the highest level, you can use all colors (prismatic effects).