Hi,
colour management is a real headache - try consulting the colour management forum chez Adobe (ignore the nasty commens about Xpress, or pretend you're using InDesign as well as Xpress).
http://www.adobeforums.com/cgi-bin/webx?14@@.eea5b31
Otherwise, I'd say this could be an issue you can solve first with Photoshop -
Open it, and in the top menu "Photoshop">"colours" check that you can chose the colour profile of every file you open, so when opening your eps file from Xpress, you can assign it the same profile that you used initially -
in the part "work space" i have (for instance) chosen for CMYK colours "Euroscale coated v2"
then below, in "colour management" I have disabled the colour management, but have selected "choice at opening" and like this I can realocate Euroscale (or any other profile in the list) to any image I open. You can also allocate a colour profile on saving a new file, or when "save as".
the same applies for RGB files.
then this may have something to do with using OPI in Quark... when you import your images, make sure that the option OPI is active, and like this, I think that Quark integrates the colour profiles incorporated in tiffs and other sorts of files. I gather from InDesign that if OPI is not activated, the page layout programme will ignore the colour profile of images included in the project. this must work the same with Xpress, so perhaps your images are losing the initial colour profile you allocated to them when they are exported as eps files. Make sure as well on exporting as eps that opi is active.
this might help keep the same colour appearance, but this is such a complex issue, and usually my prespress pals tell me not to bother with assigning profiles, as so much depends on the paper and the inks used etc. and I for one know nothing about the art of printing.
Doubtless someone else will come up with a more useful idea soon - and for my part, I'd be grateful to read what they say... help! someone !