It's remarkable how many different viewpoints there are on this subject, no? --including some that can't possibly be correct at the same time. "Coloration" seems to be like "resolution"--a (seemingly) common-sense term used with great confidence by many people as if there was general actual agreement about what it means.
There are many things that a preamp can do besides simply amplifying a signal--it can add distortion of various kinds; it can have a limiting or compressing effect (which creates distortion, too), and/or it can have non-flat frequency response, for example. There are preamps which do all of these things and more, which are hotly desired by some engineers. All I can say is that I don't go there; what boojum said above (especially the second part of his message) is exactly what I believe. Before I take a preamp out for live recording I put it up on the test bench and make sure that it has flat frequency response and that it can handle the expected signal levels without overload (soft, hard or in-between, I don't want it).
In the process I've come across one otherwise very good preamp (the Rane MS-1B) that as you turned up the gain, its low-frequency response rolled off more and more; a series of rather mediocre preamps (ART Tube MP) that can merrily pump out 25% THD before their overload LED decides to come on--or only 0.5% THD (which I don't consider audible), all depending on exactly how you set the (three!) gain controls. I've seen one "classic" "vintage" preamp (Telefunken V 72) with a boosted low-frequency response, which has an effect that some people would describe as "warm"--but you could just as easily get that effect by boosting any other preamp's response the same amount, so how much of a virtue is that, really? And I've tried two kinds of Aphex "Tubessence" equipment (a preamp and a parametric equalizer) which used a tube for coloration, but after listening for a while, I gained a real appreciation for how the original recording sounded without any of that.
I'm far from being a purist; I'm all in favor of any kind of signal processing that gives favorable results, any way you choose to define that. I'm fond of the old Carver "Digital Time Lens" which uses M/S techniques to produce some audiophile enhancements; they're especially nice on some X/Y recordings. And Peavey used to make an inexpensive rack-mount device called the "Tube Sweetener" which adds tube coloration in controlled amounts to any stereo signal. Both are fun to experiment with, and sometimes they're just the cure for what's wrong with a recording. As is careful equalization--a fundamental skill that deserves great respect.
But a clean original recording is always the best starting point. You can't remove the excess salt from a soup once it's been poured in.
--best regards