Material Input Channels
The Screen Space Subsurface shading profile does not differ much from the Lit shading mode, with the main difference being that the Metallic input has been repurposed and is not available to be used.
Base Color Input: The Base Color input is used as usual for Diffuse lighting. There is no extra Subsurface Scattering color as the Screen Space Subsurface Scatter is not supposed to change color or brightness, it is just redistributing the lighting to nearby pixels.
So, if a material should scatter in a specific color, it needs to be expressed as part of the Base Color.
The Base Color should be the final color as if the material is viewed from a large distance where the scattering cannot be distinguished from diffuse lighting.
Human skin is a thin layer that blocks a certain amount and color of light, and it covers vibrant, red-colored flesh below its surface. The visible scatter distance for light-colored human skin is about 1.2 CM.
Metallic Input: The Metallic input channel is not available when using a Subsurface Profile because the GBuffer space for the Metallic input has been repurposed to accommodate the Subsurface Profile data.
Opacity Input: The Opacity input channel is used to mask the Subsurface scattering contribution.
It works by using a value in the 0 to 1 range to allow for smooth transitions between different areas of Subsurface scattering intensity, where 0 is no scattering, and 1 is full scattering.
To better control where the Subsurface scattering is stronger or weaker, it is best to use a mask texture.
Areas in the mask Texture that have values closer to 1, or white, will have the strongest Subsurface scattering effect while areas that are closer to 0, or black, will have less of an effect.
Adjusting the Subsurface Color will help to compensate if areas get too dark. Remember that using brighter colors results in more Subsurface scattering.
Here you can see how the mask can be used to render two surface types with one material. Note that the transition is soft and not limited to triangle borders.