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// Fast and easy way to combine (additive mode) two RGBA colors with JavaScript.
// [red, green, blue, alpha] based on these maximul values [255, 255, 255, 1].
var base = [69, 109, 160, 1];
var added = [61, 47, 82, 0.8];

var mix = [];
mix[3] = 1 - (1 - added[3]) * (1 - base[3]); // alpha
mix[0] = Math.round((added[0] * added[3] / mix[3]) + (base[0] * base[3] * (1 - added[3]) / mix[3])); // red
mix[1] = Math.round((added[1] * added[3] / mix[3]) + (base[1] * base[3] * (1 - added[3]) / mix[3])); // green
mix[2] = Math.round((added[2] * added[3] / mix[3]) + (base[2] * base[3] * (1 - added[3]) / mix[3])); // blue

// Will return [63, 59, 98, 1]