YOu should do this using profiles. the -colorspace option is only used for internal storage changes with only minor secondary effects for image file saving.
See IM examples, Color Profiles
http://imagemagick.org/Usage/formats/#color_profile
Conversion of all JPEG in a folder to CMYK
- anthony
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Re: Conversion of all JPEG in a folder to CMYK
Anthony Thyssen -- Webmaster for ImageMagick Example Pages
https://imagemagick.org/Usage/
https://imagemagick.org/Usage/