CMYK and RGB are relative colour spaces, and as a result, individual values mean nothing. In RGB's case, it means the intensity of three lights, a white point colour, and a transfer function which are not specified by the values alone. In CMYK's case, it means four (or more) arbitrary ink colours as well as paper stock and illuminant.
Taken in isolation with no additional metadata, RGB values and CMYK values mean nothing, and there is no method to convert them.
In order to specify a distinct colour, values would need to be offered using another colour encoding model such as xyY, along with some additional context, or, optionally, in RGB with a clearly defined colour space. Same would go for offset work, via an associated printing colour space encoded into an ICC format.
It should also be noted that when dealing with shaders, you should not match sRGB values, for example, but rather match how the physical object reflects light relative to the lights used in the renderer's reference space. This is likely the subject of another post however.
I take my guitar, plane the note "c", this note i will record using a digital microphone. Then i take the recording on my computer, burn it on CD, put the CD into my hiFi system and play it.
It is closer to the idea of a guitar tablature; you don't know the guitar tuning, so playing the particular fret position is essentially meaningless without knowing the guitar tuning.
In much the same way, RGB values purely are intensities. What isn't communicated via an intensity value is:
- What the colour of each light is for each red, green, and blue intensity. Is the red a purple colour, the green a yellowy tone, the blue a cyany?
- What arbitrary scale the intensity is mapped to. Is it a physical ratio, a non-linear scale of one of the infinite out there, or some perceptual scientific ratio?
- What the achromatic value is intended to be. What colour are we using as the adapted achromatic colour for our notions of white?
Hence, according to the ISO, we must answer all three of these questions before we can use the term colour space.
Given an RGB colour space and the values, we can get an absolute device independent notion of what the colour in question is, and convert it to any other domain accurately.