My main concern is getting to where it looks right after weathering... I use MM OD right outta the rattle can, then start there with the weathering process, as that's basically the way it works on the prototypes..
I like the WW2/Korean War USN colors of blue as MM makes them, so they get shot right outta the can as well...
Overall though, folks that obcess over the exact shade of a particular paint needn't do so, unless that's part of their process... I've been in the Army through four different Army-wide tactical vehicle camouflage schemes (Overall OD, 4-color NATO, 4-color NATO DESERT, and MERDC), AND the Desert Sheild/Desert Storm camo, which, in our case, was so hastily applied that it was chipping off in large areas almost before they got to KKMC... We didn't even paint the Howitzers and the FAASVs. They were still in the MERDC scheme or (the newer ones) NATO scheme with CARC, although the soft-skins got repainted with the Desert Sand CARC in most cases...
This next nit is more about technique than color, but I gotta say something about it anyway..
There was an article in FSM some months back that featured a track with varying shades of the base-color that sticks in my mind... While it looked fine in the magazine, I suspect that using the Mk I Eyeball, in person, that paint would have looked like what it was, three or so different shades of paint.. I've seen that before, in oerson, and gotta say that's not only a lot of work, it's a lot of work for nothing, because it doesn't (to me) look like the same color of paint under various lighting conditions...
Frankly, it looks fine if the primary purpose of the paint job is for a magazine article or the like, since you're showing a 3-D object in a 2-D format, but it doesn't look right if the lighting ever changes angles and intensity... The only place this type of base-color painting would be desirable is if it's in a shadow-box, where the viewing angle is restricted to one, and the lighting is controlable, IMNSHO... "Shading" a model vehicle of aircraft is just asking for trouble on the contest-table... A builder has no more control over the lighting and angles used than he/she has over the weather outside the venue... However, I digress..
Getting the "exact" shade of a particular prototype is a pipe-dream.. I have a mild color-vision problem, for one thing, and I judge at contests too, lol... (Red-Green Color Discrimination is what the doc called it and before y'all start, IT HAS NOTHING TO DO WITH POSSUM LODGE OR THE LOCALS LIVING AROUND THE POSSUM LAKE, CANADA AREA)...
Even if you do get it, using the same paint from the same can that you got from the factory isn't gonna look quite right... So, TLAR applies in most cases and will go a long way in allowing you to build and paint more, and research a little less... Most model paint manufacturers went to a lot of work getting the paint the right "Scale" color in the first place... Duplication of effort is rather a waste of bench-time (if such a thing matters to you, that is) While I don't claim to know about the atmospheric influence on paint shades that're scaled down, I know what "looks right" in my eyes... So I go with that...
Another thing I touched on, but didn't expound upon, is the fact that one can look at a line of M60A3, M109A3s, or Ex-RAF P-40s from the Umpteenth Fighter Sqn, Eleventeenth Fighter Group, Eightish Air Force and see ten different "shades" of colors, all dependant on what day of the week they were initially painted, how long they've been out in the weather, the amount of salt-air, desert winds, arctic temperatures they were exposed to, etc, etc., etc., and if they were painted with factory paints, or locally procured shades (VERY common in the 8th AF in England, especially with the OD used)..
So the bottom line for me is, Don't sweat it... Judges don't carry color-chip booklets with 'em (same thing goes for the Pilot's Operating Handbook or the Dash 10if they do, they got issues, are probably still living in their parent's basement and you're screwed anyway... Might as well leave the seam in that metal barrel...)
Oh.. One more thing, and then I'll shut up about it... Being a shade or two off, or having the curvature of the camouflage demarcation line a scale two inches too low is not an issue... However, just because there's a bit of wiggle room it doesn't mean you can conjure up totally ridiculous patterns and colors, unless you're doing, on purpose, a "What If?"-type of build or contest, and you need to be able to explain that if asked... And don't try to pass off Olive Green as Olive Drab, Dark Grey as Panzergrau, light blue as RLM Lichtblau or Khaki as Tan...
It's kind of like Creative Gizmology, wherein the Gizmologist doesn't try to duplicate, bolt-for-bolt, nut-for-nut, types of added details, but suggests them, while still maintaining a good bit of Imagineering... Suggestion, rather than Duplication, is the key... It's quite acceptable, it's far less money and time, and it's, above all else, it's YOURS...