[Cleanup] Use p5.color built-in functions to adjust RGBA values in rainbow effect

[epeach]

In order to get the rainbow effect to use palettes as quickly as possible, I manually adjusted RGB values of the p5.color object by incrementing values in an array.

dance-party/src/Effects.js

Line 177 in 80c5729

let [red, green, blue] = paletteColor.levels;

We should move to using the built-in p5.color functions to adjust RGBA. https://p5js.org/reference/#/p5.Color