JSFiddle - React, Tailwind, and code Playground

HTML

<canvas id="primary"></canvas>

CSS

#primary {
	display: block;
	border: 1px solid gray;
}

JavaScript

function color(r, g, b) {
  var args = Array.prototype.slice.call(arguments);
  if (args.length == 1) {
    args.push(args[0]);
    args.push(args[0]);
  } else if (args.length != 3 && args.length != 4) {
    return;
  }
  return "rgb(" + args.join() + ")";
}

function drawPixel(x, y, fill) {
  var fill = fill || "black";
  context.beginPath();
  context.rect(x, y, 1, 1);
  context.fillStyle = fill;
  context.fill();
  context.closePath();
}

var canvas = document.getElementById("primary");
var context = canvas.getContext("2d");

canvas.width = 256;
canvas.height = 256;

for (var x = 0; x < canvas.width; x++) {
  for (var y = 0; y < canvas.height; y++) {
    var r = 255 - y;
    var g = 255 - x - y;
    var b = 255 - x - y;
    drawPixel(x, y, color(r, g, b));
  }
}