Thanks a lot guys, this is very valuable information for me and I appreciate the time and trouble you've taken for no reward. This is what "online community" is all about. Hope to be able to contribute likewise myself eventually, but I've a helluva lot to learn!
Meanwhile, one more question: I've been using Lightroom, but should I get Photoshop for this sort of work? For instance, using the example of a pale blue affected by a warm paper tone, can that be corrected as easily in LR as in PS? Or might PS be needed not for that but for other likely painting-copying scenarios?
(My situation: with some interest and experience in photography, a certain amount of capital, and looking for a small scale business opportunity, an artist friend suggested fine art printing, and I ran out and spent loads of money without doing as much research as I should have, but I'm now quite committed, and this will be my main occupation for the foreseeable future.)