Real color is a quite complex subject. In essence color is something that happens somewhere between your cornea and brain. Simplifying this a bit color is what you sense when some photons interact with the three color sensing structures in your eye. Other definitions exist but they fail on many levels.
Photons can reach your eye trough many processes. They can reach your eye directly from the light source, or they can bounce on the way. When they bounce your actually seeing the color that the surface did not absorb. Whereas when you emit light you see what was sent unless something obscures the light on the way. The photon can also reach your eye trough other processes, some which are weird and obscure but that's another subject.
Because we have 2 primary systems for imaging in design, we also have 2 base models: additive and subtractive. One deals with emissive media like monitors. The other with reflective media like paper, but also works for things like stained glass. These models are inverses of each other: RGB is the inverse of CMY; mixing RG gives you Y and so forth.*
Manually mixing color has some other challenges. Your pigments may not react with each other chemically, or anything can happen. The pigments also need to be pure, and not have too big of a spread in what they remove from the spectrum as the eyes sensors overlap slightly. I find it extremely unlikely that you will find paint that fulfills this requirement easily.
It's also important that your lightning conditions need to be good as it affects how the subtraction happens. It's possible to design colors that change when you go indoors under artificial light, as compared to direct sunlight (This is called metamerism).
Now comes the mindblowingly hard part. In order to mix the right color you need to measure the colors. See, RGB values themselves are just signals; they do not represent colors without you knowing how the device displays the color. Each monitor behaves differently, and so do your pigments.
* Interesting fact: The ordering, RGB and CMY, are not mistakes. Any of the two colors in a triplet combined makes the corresponding color you didn't choose in the corresponding inverse triplet, so G + B = C and C - Y = G. The fact that the colors don't display as pure is because the computer tries to mimic available pigments, failing miserably.