Grayish colour would be something like 40% 40% 40% 0%
Of course it depends on the model's skin, for example a water elemental will only turn dark blue and an orc would turn dark green. All units that are about white normally will turn gray when the vertex colouring is set to those % I mentioned before.
And btw. setting animation speed to 0 doesn't disable the unit's movement, so I recommend pausing or stunning the unit as well