code: public void setA(final float a) {
          if (a < 0 || a > 1) {
              throw new InvalidValueException(
                      "a paramater accept values only from 0 to 255.");
          }
          this.a = a;
          rgba = "rgba(" + r + ", " + g + ", " + b + ", " + a + ")";
          if (getStateChangeInformer() != null) {
              getStateChangeInformer().stateChanged(this);
          }
      }
docs: to set alpha. The default value is 0.
  
  @param a
  @author WFF
  @since 1.0.0