intel i5 6400, windows 10 64
blender 2.78.5 f3d9f0c
Short description of error
The color of the diffuse bounced light of Principled BSDF shaders with Subsurface Scattering is wrong. The color is always that of the Base color input, instead of the proper mixture of both Base color and Subsurface scattering color.
Both balls on left are materials using Principled BSDF, while the two on the right have better behaviour with a mixing of diffuse and subsurface scattering in the node editor. In all materials the diffuse is cyan (0,1,1) while the SSS is pure red (1,0,0)
There's another problem that might be related. You can see that the bottom right grey ball, which uses a mix shader of diffuse and SSS at 50% has a red backscatter light, while the bottom left ball, the one with Principled BSDF and SSS also at 50%, has just a grey backscatter. It seems that, in Principled BSDF, the color of the SSS backscatter is the mixed color of both Base Color and SSS color inputs, instead of just the color of the SSS color input itself, which seems to defeat the purpose of SSS in the first place (although using SSS at 100% and controlling the radius of each color works fine, but it's just a single scatter) Tell me if I should report it in another bug report, or if it's not a bug, where can I discuss this behaviour with the apropiate developer.
Exact steps for others to reproduce the error