I recently acquired an Epson 1400 converted to CISS. I'm printing show posters created in Illustrator CS5 on French Paper poptone 100# cover in Sweet Tooth. My colors aren't matching (they look "dumbed down").
You're printing CMYK on uncoated colored paper, probably from a RGB file. That wont work too well. Either print on white paper or over saturate your colors to compensate for printing on the colored stock. But you still may run in to issues because of the color / print profile you are using and UCR.
if you so wish there is a medium you can buy from an art supply store online or in store. They are made by Golden and are Digital Ground
here is a link for Utrecht- Utrecht
I haven't used it but I am thinking about it.