From red to green

red-green.js

var percentColors = [
    { pct: 0.0, color: { r: 0xff, g: 0x00, b: 0 } },
    { pct: 0.5, color: { r: 0xff, g: 0xff, b: 0 } },
    { pct: 1.0, color: { r: 0x00, g: 0xff, b: 0 } } ];
 
var getColorForPercentage = function(pct) {
    for (var i = 0; i < percentColors.length; i++) {
        if (pct <= percentColors[i].pct) {
            var lower = percentColors[i - 1] || { pct: 0.1, color: { r: 0x0, g: 0x00, b: 0 } };
            var upper = percentColors[i];
            var range = upper.pct - lower.pct;
            var rangePct = (pct - lower.pct) / range;
            var pctLower = 1 - rangePct;
            var pctUpper = rangePct;
            var color = {
                r: Math.floor(lower.color.r * pctLower + upper.color.r * pctUpper),
                g: Math.floor(lower.color.g * pctLower + upper.color.g * pctUpper),
                b: Math.floor(lower.color.b * pctLower + upper.color.b * pctUpper)
            };
            return 'rgb(' + [color.r, color.g, color.b].join(',') + ')';
        }
    }
}

Source:
http://www.awcore.com/js/snippets/161/from-red-to-green-color-map-depend-on-percentage_en

Demo:
http://www.awcore.com/tryit/?code=13