I'm sure this is really simple and I'm drawing a giant blank, but how do you set the result of a function as a global variable?

Example, I want to set the first "color" in array "colors" as global variable "color" (I know the example doesn't make much practical sense, but it's just to illustrate my question):

var color = "";

function selectColor () {
    var colors = ["blue","red","green","yellow"];
    var color = colors[0];
    return color;
}

window.onload = function () {
    selectColor ();
    alert(color);
}