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you can do 2 passes, one for white, and one for black.

the grey mask then is the inverse of the or'ed black and white mask. example:

Mat_<uchar> m(1,5); m << 30,110,170,220,240;

Mat b = m < 100;
Mat w = m > 200;
Mat g = ~ (b|w);


cout << b << endl;
cout << g << endl;
cout << w << endl;

.

[255,   0,   0,   0,   0]
[  0, 255, 255,   0,   0]
[  0,   0,   0, 255, 255]