Color Interpolator - when you need to find a color in a colorstop-defined gradient.

ColorInterpolator.js

// usage: prime with a set of color stops, then ask for colors in the range.
//
// colors = ['#37d78a','#40be8f','#49a594','#538d9a','#5c749f','#655ba4'];
// colorInterpolator = new ColorInterpolator();
// colorInterpolator.prime([
//  { value: 0.0, color: colors[0] },
//  { value: 0.2, color: colors[1] },
//  { value: 0.4, color: colors[2] },
//  { value: 0.6, color: colors[3] },
//  { value: 0.8, color: colors[4] },
//  { value: 1.0, color: colors[5] }])
//
// colorInterpolator.color(0.35)

function ColorInterpolator() {
  this.interpolators = null
  this.start = { value: 0, color: [0,0,0] }
  this.end = { value: 1, color: [255,255,255] }
}

ColorInterpolator.prototype = {
  prime: function(colorStops) {
    var that = this
    $.each(colorStops, function(i,colorStop) {
      if (typeof colorStop.color === 'string') {
        colorStop.color = that.rgb(colorStop.color)
      }
    })
    if (colorStops.length > 2) {
      this.interpolators = []
      for (var i = 0; i < colorStops.length-1; i++) {
        var newStops = [colorStops[i],colorStops[i+1]]
        this.interpolators.push(new ColorInterpolator().prime(newStops))
      }
    } else if (colorStops.length == 2) {
      this.start = colorStops[0]
      this.end = colorStops[1]
      return this
    } else if (colorStops.length == 1) {
      var newStops = [[that.start,colorStops[0]], [colorStops[0],that.end]]
      this.prime(newStops)
    }
  },

  rgbColor: function(value) {
    var interpolators = this.interpolators
    if (interpolators) {
      if (value < interpolators[0].value)
        value = interpolators[0].value
      else if (value > interpolators[interpolators.length-1].value)
        value = interpolators[interpolators.length-1].value
      for (var i = 0; i < interpolators.length; i++) {
        var interpolator = interpolators[i]
        if ((interpolator.start.value <= value) &&
            (interpolator.end.value >= value)) {
          return interpolator.rgbColor(value)
        }
      }
    } else {
      var start = this.start
      var end = this.end
      var fraction = (value-start.value)/(end.value-start.value)
      var red   = Math.floor(((end.color[0]-start.color[0])*fraction)+start.color[0])
      var green = Math.floor(((end.color[1]-start.color[1])*fraction)+start.color[1])
      var blue  = Math.floor(((end.color[2]-start.color[2])*fraction)+start.color[2])
      return [red,green,blue]
    }
  },

  toHex: function(d) {
    return  ("0"+(Number(d).toString(16))).slice(-2)
  },

  color: function(value) {
    var rgb = this.rgbColor(value)
    return "#"+this.toHex(rgb[0])+this.toHex(rgb[1])+this.toHex(rgb[2])
  },

  rgb: function(value) {
    return [parseInt(value.substr(1,2),16),
            parseInt(value.substr(3,2),16),
            parseInt(value.substr(5,2),16)]
  }
}