I've changed the /system/build.prop line where the low_bit_enabled was to
And after clearning both normal cache and dalvik, and even doing a factory reset (Which kept my build.prop), I still can't manage to get grayscale on ambient mode.
As a reference, I included the crappy lowbit mode, and what I'd like to see.
I'm not sure If what I'm missing is a watchface that doesn't strip away grayscale when in ambient, or another setting.
It does limit which watchfaces we can use in ambient (that look good), but at least it has limited colors in ambient mode.
I'm currenlty using the Spotlight watchface and it looks exactly the same in, ambient and active.