After some testing, I've found the same thing. It looks like the font colors in some memo fields are being reset to defaults by the XML parser. The FIELDFONT tag does not affect the font color on these fields.
Affected fields are: Custom, tagline, comment, and category.
When you go into EDIT MODE, the requested colors are used in the display, so the color information is being used, but on exiting EDIT MODE, the colors revert to black regardless of any other settings. I've attached a screen shot of the edit mode screen.
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