posterize TIFF-files: how to prevent change of color-depth?

[fmw42]

ImageMagick default depths are defined by the Q level of the install. If using Q16 (default), then if the format supports 16-bit depths, then it will be 16, but if not then you will get 8-bit depths. To enforce 8-bit depths use -depth 8 or use a Q8 install.

[MaxiPunkt]

"-depth 8" has no effect in this case
"- type truecolor" is the key to solve the problem

[fmw42]

Only since you want 24-bit total color and want more than 256 colors. Otherwise, if less than 256 colors you would get 16-bits total (palette) without -depth 8 and 8-bits total (palette) with -depth 8.