Get two seperate textures, one for each colour of grass (so one light green one dark green)
Next, create an alpha mask. It should end up looking like the picture above with alternating black/white instead of light green/dark green.
Then you can use the alpha mask to tell Unreal where each green should appear with a linear interpolate node (Lerp). Plug light green into A, plug dark green into B and plug the mask into the Alpha. Like this
I used an alpha mask from my mosaic tile material but the principle is still the same
Not sure this is the most efficient way, but this is how I would do it.