Ok I found what could , perhaps be the solution , I edited the base material and I added this :
I will write it here in case others might incur in the same issue :
https://i.gyazo.com/a000d3043153299a560e8bf77afee592.png
Then I see that the megascan material of the grass is actually two , not sure how are implemented but editing both I could in one edit the distant color grass and the other edit the close color grass, so working on both I could achieve this :
https://i.gyazo.com/97a56926cf8ecc759463b84656fc868a.jpg
I still think there is an issue in the transition color and I am not sure where to go to edit this , but I think looks better than earlier already no?