Why the grass looking too black?

I am using Unreal 4.25 with RTX enabled. For grass i am using Megascan Grass, Problem is after painting the grass using foliage its looks like this when the camera at certain distance

But when I close the camera the grass looks like this

A combo of things.
bad colors, auto exposure, bad reflectivity, bad LODs possibly.

To know what exactly the issue is you have to break down and re-build the asset your self bit by bit.

Start with the textures and check if sub surface is active.
move to mesh and look at vertex normals. Are they pointed up?

Maybe this first.
Check your sceene lighting. Is there any? Is there a skylight?Is it movable. Etc.

MostHost LA, are you sure you can use subsurface with RTX enabled?
If yes, can you please elaborate a little bit more.

Its still unresolved actually

So your base color would need to be lighter than you think to make up for it, and the grass won’t really ever look realistic unless you take it out of the ray trace pipeline by using a different material or setting it up to defer render.

That said. If you made sure the light exists and is fine, the next part is to look at the LODs and MIP values of the texture.
maybe even overwrite the LOD material with a lighter one.
It really depends on what the material is doing.
is it even setup for ray tracing since its a mega scan asset?