[scribus] Color problem when doing imposition

[Xavier Colmant]

I am setting up a newspaper with Scribus. I work on A4 pages but the
final result is professionally printed on A3 sheets. Before I was
doing the job on XP. I switched to Linux. To do the imposition in XP I
was using Multivalent, a java application. I was using Multivalent
because it was the only application usable with XP which did not do
color shifts during the imposition process (my professional printer
uses CMYK while my PC works with RGB). Since I switched to Linux
Multivalent shifts the color balance towards the Red. I tried doing
imposition directly with Acrobat but I still get a redish colorshift
(albeit less pronounced with Acrobat).
I'm totally at lost with this colorshift. The only thing I found is to
produce a PDF with the A4 pages correctly imposed for my professional
printer. He then combines A4 pages into A3 pages with Quark Xpress on
his Mac based machines. He's a friend, so the issue is not deadly
(we're both anarchists : Content is more important than commercial
dealings), but I would like to spare him extra-work. Does anyone has
an idea about the way to correct the issue ?