https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Applying_styles_and_colors

https://developer.mozilla.org/ja/docs/Web/API/CanvasRenderingContext2D/fillStyle

すべて文字列で設定してる！

ctx.fillStyle = 'orange';
ctx.fillStyle = '#FFA500';
ctx.fillStyle = 'rgb(255, 165, 0)';
ctx.fillStyle = 'rgba(255, 165, 0, 1)';

ctx.fillStyle = `rgb(
${Math.floor(255 - 42.5 * 3)},
${Math.floor(255 - 42.5 * 5)},
0)`;