Last month, I tried to match the colour of transfer I designed to a vallejo paint I used, by adjusting the RGB values of the printed colour, adding a stroke of the paint on each print attempt to judge how close I was.
It was terrible; I made about 20 tries before settling on the least bad option. Only to discover two things;
1) Printing on transfer paper gives different results than printing on regular paper.
2) (and here is where it gets relevant) When comparing the near-match of my choice under daylight as opposed to under the desk lamps I had been using, the colour turned out to be
way off again!
See; because the reflectivity, structure and absorbtion of each medium is different, each reflects light differently as well, resulting in vastly different colours under different lighting conditions.
So, just like it's impossible to match monitor colours uniformly to paint colours (see the LAF Legion thread over in Future Wars; it's stickied), it's also nigh impossible to match printed colours to paints.
To compound the issue, manufacturers can change ingredients or even recipies without warning, so even two pots of paint of the same colour can have differences, ranging from minute to glaring.
Try
Encycolorpedia for exhaustive information on Vallejo colours in a whole range of different types (including CMYK).
But you've been warned that such a pursuit will lead to certain madness though...