Use getComputedStyle to get an RGBA value from any supported CSS color.

getComputedColor.js

(function(){

	var getColor = function(c) {
		var temp = document.createElement("div");
		var color = {r:0,g:0,b:0,a:0};
		temp.style.color = c;
		temp.style.display = "none";
		document.body.appendChild(temp);
		var style = window.getComputedStyle(temp,null).color;
		document.body.removeChild(temp);

		var recol = /([\.\d]+)/g;
		var vals  = style.match(recol);
		if (vals.length>2) {
			color.r = parseInt(vals[0])||0;
			color.g = parseInt(vals[1])||0;
			color.b = parseInt(vals[2])||0;
			color.a = Math.round((parseFloat(vals[3])||1.0)*255);
		}
		return color;
	}

	console.log(getColor("blue"));
	console.log(getColor("rgba(0,0,255,0.5)"));
	console.log(getColor("rgb(0,0,255)"));
	console.log(getColor("#0000ff"));

})();