main.js

var rgb2hsl = require('./rgb2hsl')
var palette = require('./palette')(options)

//e.g.
console.log( rgb2hsl([255,0,255]) )

palette.js

function Palette(options) {
  //this line allows somebody to just require('palette')(opts) instead of 
  //always needing to call new Palette(opts)
  //it's optional, but I find it leads to cleaner code
  if (!(this instanceof Palette)) return new Palette(options)
  options = options||{}
  
  ... do something here ...
}
module.exports = Palette

rgb2hsl.js

module.exports = function rgb2hsl(rgb) {
  ...
}