function red(color, value){
  color = color.rgba;
  if (value) {
    return rgba(
      value,
      new nodes.Unit(color.g),
      new nodes.Unit(color.b),
      new nodes.Unit(color.a)
    );
  }
  return new nodes.Unit(color.r, '');
}
Return the red component of the given `color`,
or set the red component to the optional second `value` argument.

Examples:

   red(#c00)
   // => 204

   red(#000, 255)
   // => #f00

@param {RGBA|HSLA} color
@param {Unit} [value]
@return {Unit|RGBA}
@api public