The Mac Imagewriter driver was never updated to print in color? That would be surprising considering the Imagewriter LQ driver could print in color and was a backwards compatible printer (extended the IW II ESC command set), but good luck finding a ribbon for one of those!

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Correct, sadly.  In order to print in color you need MacPalette II.  

http://www.jagshouse.com/colorimagewriter.html

The Imagewriter driver found in OS X and Linux comes from Ghostscript, that driver never had color support added to it. Ghostscript is mostly open source, so adding color support is possible. One could "borrow" code from Ghostscript's Epson LQ driver to add color support.

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That would be fantastic!  I wonder how much work it would take.  The IW II uses a C.ITOH printing engine so if any other related printers have color support then that code might also work.