add(r, g, b, a) {
    return new RGBA(
      this.r + r
      , this.g + g
      , this.b + b
      , this.a + a);
  }
Add r,g,b,a to the current component values.

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