If source image is a 8-bit depth palette PNG that only
uses 4 unique colours but does not use black colour,
reducing bit depth to 2 and palette length to 4 colour
via switches "-reduce -brute -rem alla -bit_depth 2
-plte_len 4" causes PNGCrush to turn one colour to
black (see attachment acme). In addition, using
-reduce switch does not reduce output images bit depth
or palette length in such case.
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