Windows 64 Pro, updated. AMD FX(tm) 8120 Eight Core 3.10 Ghz, NVidia GeForce GTX 760
Broken: (2.78 Release Candidate)
Worked: (prior version 2.77a+)
I noticed that the color picker was "poor" low quality display, totally dark on the Value bar regardless of the color chosen, and I tried the other choices in the User Preferences, none were all that much better. The color display in the square version reminded me of old 8 bit color, with large boxes. Nothing I did seemed to change the display to what I remember on previous versions.
Exact steps for others to reproduce the error
Open 2.78, choose a material for a default object, (cube) then select the Diffuse color. The picker comes up with a completely black Value Bar and no sliding up or down makes a difference.