Return the CSS color for the specified CSS attribute. rgb, 3 digit (like `#fff`) and valid values
are convert to standard 6 digit hex color.
@param {String} attr The css attribute
@param {String} defaultValue The default value to use when a valid color isn't found
@param {String} [prefix] defaults to #. Use an empty string when working with
color anims.
function(attr, defaultValue, prefix) {
        var v = this.getStyle(attr),
            color = prefix || prefix === '' ? prefix : '#',
            h, len, i=0;

        if (!v || (/transparent|inherit/.test(v))) {
            return defaultValue;
        }
        if (/^r/.test(v)) {
             v = v.slice(4, v.length - 1).split(',');
             len = v.length;
             for (; i<len; i++) {
                h = parseInt(v[i], 10);
                color += (h < 16 ? '0' : '') + h.toString(16);
            }
        } else {
            v = v.replace('#', '');
            color += v.length == 3 ? v.replace(/^(\w)(\w)(\w)$/, '$1$1$2$2$3$3') : v;
        }
        return(color.length > 5 ? color.toLowerCase() : defaultValue);
    }