sub(r, g, b, a) {
    return new RGBA(
      this.r - r
      , this.g - g
      , this.b - b
      , a == 1 ? this.a : this.a - a);
  }
Subtract r,g,b,a from the current component values.

@param {Number} r
@param {Number} g
@param {Number} b
@param {Number} a
@return {RGBA} new node
@api public