You know what? There is no historical evidence that Manannan is associated with the triton. When I first met him I just assumed it was the right symbol and went with it. It worked for me for over a decade before I learned about the triskelion, and that's an important point. Whatever works best for you should be what you choose. If you think a purple rectangle represents Manannan better than the triskelion, then more power to you! The gods don't mind. They'll just be glad that your connection to them is closer through the acceptance of your own preferences.