The Clay default color was redefined in the last review of the YafaRay Color Spaces system, where I implemented a more correct sRGB color management.
Currently the Clay default color is exactly middle gray in the Linear color space (0.5,0.5,0.5).
However, Blender color picker shows values in a slightly confusing way:
* Floating point components of the color are shown in linear color space (0.5,0.5,0.5)
* Hex color components are shown already converted to the selected Color Space in Blender. For example, if you select color space sRGB, you will see it's 0.5,0.5,0.5 but the hex value is BCBCBC. If you select color space "none" you will see the same floating point values 0.5,0.5,0.5 and the hex value 808080
So, the current implementation of color spaces and clay default color is supposed to be mathematically correct to be 0.5 in the linear color space, which is the space where all rendering calculations take place.
Of course, I could be wrong but samo and me worked on this for quite some time and agreed most of those changes and definitions.