That TIJ cars were distinguishable only by color was made possible by filming in color. A BW movie would not make a film requiring distinguishing colors that filmed as equal shades of grey.

In any case, any such colorizing system would be designed to accept a bit of guidance here and there from the artist. This is much like when an OCR'd document needs a bit of touch-up.

And even if it wasn't perfect, many BW movies would be made much more watchable, like the 1927 Wings, which is crying out to be colorized (and have a soundtrack added).