code: function inerpolateColorsFixedAlpha(col1, col2, amount, alpha) {
    let col1rgb = parseColor(col1);
    let col2rgb = parseColor(col2);
    return RGBobj2string({
      r: Math.min(255, col1rgb.r * amount + col2rgb.r * Math.max(0,1-amount)),
      g: Math.min(255, col1rgb.g * amount + col2rgb.g * Math.max(0,1-amount)),
      b: Math.min(255, col1rgb.b * amount + col2rgb.b * Math.max(0,1-amount)),
      a: alpha
    });
  }
docs: Interpolate two colours and set the alpha value.