CanvasRenderingContext2D: fillStyle プロパティ

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

const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
ctx.fillStyle = "blue";
ctx.fillRect(10, 10, 100, 100);

<canvas id="canvas" width="150" height="150"></canvas>

const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
for (let i = 0; i < 6; i++) {
  for (let j = 0; j < 6; j++) {
    ctx.fillStyle = `rgb(
        ${Math.floor(255 - 42.5 * i)}
        ${Math.floor(255 - 42.5 * j)}
        0)`;
    ctx.fillRect(j * 25, i * 25, 25, 25);
  }
}

setFillColor(color, /* (optional) */ alpha);
setFillColor(grayLevel, /* (optional) */ alpha);
setFillColor(r, g, b, a);
setFillColor(c, m, y, k, a);