Why do you want to use subsurface? ^^ -> you should use the two sided foliage shading mode (that’s the new foliage shading system)
It’s grey because you have to connect your diffuse map with the subsurface link to give it a colour.
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Subsurface - Subsurface Profile**
The ability to rendering realistic looking human skin is a must for any modern Video Game engine. To fulfill this need, Unreal Engine 4 (UE4) now offers a shading method specifically for skin or wax like surfaces called Subsurface Profile. While the Subsurface Profile shading model has similar properties to the Subsurface shading model, its key difference is in how it renders. SubsurfaceProfile uses a rendering approach that is based in Screen Space because this helps to better display the subtle subsurface effects seen in human skin where backscattering is a secondary effect only seen in few cases like ears for example. The following document will cover the what a Subsurface Profile is and how you can use them in your work.