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