It’s caused by the Global Illumination in UE4 which can sometimes be too saturated for bright colors.
An easy solution to this is to use a ‘Lightmass replace’ node in your material setup between the grass texture and the base color input. Plug the texture into the Realtime input and for the Lightmass input you can use a white color, or a very light green if you still want some Global Illumination.
What this node will do is it will use the Realtime input for the diffuse look for your material, and use the Lightmass input when simulating lighting being bounced off the material.