JSFiddle - React, Tailwind, and code Playground

JavaScript

// We want to know the rgba values for this color name:
var testColor = "blue"

// make a canvas
var canvas = $('<canvas width="100px" height="100px">');

// optional: display the canvas
var body = $(document.body);
canvas.appendTo(body);

// draw a rectangle on the canvas
var context = canvas[0].getContext("2d");
context.beginPath();
context.rect(0,0,100,100);
context.fillStyle = testColor;
context.fill();

// get the canvas image as an array
var imgData = context.getImageData(0, 0, 10, 10);
// rbga values for the element in the middle
var array = imgData.data.slice(50*4, 50*4+4);
// convert the opacity to 0..1
array[3] = array[3] / 255.0;

$("<div>The rgba for " + testColor + " is " + array + "</div>").appendTo(body);