So, after watching the Anime, I noticed that all the people, who inherited the power of the Founding Titan, eye's change from their original colour to a brilliant purple colour, whether they be royal blood or not, as evidence of Grisha Jaeger. It should noted that the holder's eyes change colour almost immediately after consuming the previous holder, as evidence by Grisha again and Frieda Reiss. Based on what is seen and known thus far, it would make sense that the purple eyes of the holder would signify that they are in control of the Founding Titan, when compared to other Titan Shifters, like Annie and Reiner.
Evidence A -- here -- Look under the Titan Tab in the character details box.
This raises yet another question, why didn't Eren's eyes change colour after he ate Grisha in the Woods? Is he not affected by the power of the Founding Titan? Or does the power need to be "activated", like the Ackerman clan, in order to use it at it's full potential?
I don't recall seeing anything about this in either the Anime or the Manga, but I could be wrong and maybe missed something yet again.