pointless practicing your colour mixing, thats like practicing to hit treble 20 only to find on the day you need treble 16.
I see where you're coming from but I don't totally agree.
Even if you go to toys r us and get some basic paints in basic colours, mixing them to get the desired colour does give you some confidence, that's what I did. But I understand what Derek says because practicing to get the perfect 'cream' is pointless when every sofa will be a variation on 'cream.'
I was lucky in that we ahve a hair dresser friend who let me change her green sofas (in the shop) to a charcoal grey. To her it didn't matter what colour black it was as she wanted them changed, so perfect colour wasn't important. This job took over 6 hours to complete (3 & 2 seater) so be prepared for a long time to do these jobs.
BTW I didn't charge her much, as it was as much useful practice for me as it was for her to get them changed, and if it all went wrong, it didn't matter.