This mod replaces most of the level objects' shaders with variants that rotate the textures around the colour wheel.
If you want to use it as a cool PaletteSwapper alternative, just set the "phase angle" or enable per-room or per-area phase randomization. Note that since the phase angle setting is in degrees setting it to 360 wraps around back to vanilla.
If you want it to make everything majestic rainbows, use the X and Y and Z settings to control the gradient strength in each of the three axes. The Z rainbow setting in particular colors objects based on how deep they are into the background. The effect can also be animated using the "Animate Speed" slider.
Thanks to BeesNation for making the original HueShifter for Hollow Knight.