Why didn't Eren's eyes change colour when he inherited the Founding Titan?

Question

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.

Answer 1

If you are not part of the Reiss Family, the founding titan's power impacts eye colour on a case by case basis.

According the the Attack on Titan Wikia:

When members of the Reiss family inherit the Founding Titan, they are affected by Karl Fritz's will, and at times, their normally light-colored eyes darken and emit a glow.

However, there seems to be no universal rule outside of the Reiss family. The eye colour aspect seems to be tied in with how it affects memory and will but as far as I can tell there is nothing official/canon so all we can do is speculate.

So here is my speculation:

Inheriting the founding titan and will of another is what changes your eye colour. This explains the Reiss Family as every one of them inherited Karl Fritz's will. But (with only a few extra steps) this also explains Grisha Yaeger.

So after eating Frieda Reiss, clearly Grisha did not inherit her will, instead I think his eye colour change was because of Eren Kruger. Kruger had done some horrible things to Grisha but in his final moments he defected, pushing his comrade off the wall to save Grisha. He then went on to justify all his bad deeds and answer any questions, explaining about the history of the founding titan and King Fritz's goal urging him to help the with the restoration project. He then injected Grisha with the titan serum and allowed himself to be eaten. Grisha did go on to help the cause, starting a family and continuing to investigate the ongoings of Paradise Island. My interpretation is that the moment he ate Frieda was the point of no return, where symbolically he truly inherited Kruger's will and thus his eye colour changed.

To answer your actual question:

Finally we have Eren Yaeger who's eyes remain green throughout. I believe his eyes remain the same colour because of his overwhelmingly powerful determination, ie. even after inheriting the founding titan his will remained his own.

Answer 2

Update: 2021/11/01 As Season 4: Episode 5 Near the end of the show, eyes color change again, and I presume founding titan power can be control by Eren.