I know Houdini's MaterialX implementation doesn't really support OCIO yet, but is there any way to get the color picker to behave correctly?

Say I have a white environment light and a grid with either Karma Principled or MatX Standard material. Both materials set to 100% roughness, 50% gray base color. OCIO environment variable, render saved to EXR.

Principled: view the EXR with OCIO sRGB view transform and the grid reads as 0.5, 0.5, 0.5.
MaterialX: same view setup, the grid reads as 0.65, 0.65, 0.65.

Long story short, Principled reads the color picker differently than MatX Standard. Is there a way to fix this?

They look identical to me. Are you sure you made the shaders the same? If one of the shaders has spec and the other doesn't, they won't produce the same value, but 0.65 vs 0.5 is a big difference.

The color picker is a Houdini widget, it's not influenced by was node it belongs to or what shader is using it. There's some OCIO integration in the color picker, but it mostly just makes it pick (eye dropper picking) incorrectly no matter what. It's only useful for displaying the numeric values in the current space. Entering numeric values in a space other than 'scene_linear' isn't possible.

If the shaders are configured the same pure diffuse reflectors, an albedo of 0.5 with an irradiance hemisphere of unity gain should produce a scene referred intensity of 0.5. Color space doesn't enter into the equation at all.

Aha, I see the source of the issue now. The shaders handle specularity, roughness and (maybe) IOR a bit differently it seems. Indeed it has nothing to do with the color picker.

If I juggle around the settings a bit I can get a match: Principled base color 0.2, reflectivity 1, roughness 1 = MatX Standard base color 0.2, diffuse roughness 1, spec 1, spec roughness 0.85.