code: private String rgbaColorStr(Color c) {
          StringBuilder b = new StringBuilder("rgba(");
          double alphaPercent = c.getAlpha() / 255.0;
          b.append(c.getRed()).append(",").append(c.getGreen()).append(",")
                  .append(c.getBlue());
          b.append(",").append(transformDP(alphaPercent));
          b.append(")");
          return b.toString();
      }
docs: Returns a string representing the specified color in RGBA format.
  
  @param c  the color ({@code null} not permitted).
  
  @return The SVG RGBA color string.