What is not expressed in saying "RGB" or "CMYK" are the other critical facets that define what they are referencing. In RGB, what are the colours of the three lights and what is the combined white point? In CMYK, what are the colours of the inks, the paper colour / type, and the illuminant in question? The mixture of the unsaid contexts are what define how colours result to a standard observer.

In the case of Lab, one must take care to assert that the same formulation has taken place before comparing values. For example, it isn't enough to just say "RGB" because of the facets above. What are the colours of primary lights in the source RGB space? The destination you are comparing? What are the white points? What is the white point assumed in the formulation for the Lab conversion?

Only when all of the contexts are identical do the values take on meaning, and a valid comparison within the limits of the encoding model is possible.

TL;DR:

* Make sure you are aware of the primaries of the RGB spaces being compared with Lab.

* Make sure your white points are aligned on the RGB spaces being compared with Lab.

* Make sure your RGB spaces are aligned to the white point of the Lab formulation in question.

* Make sure you are linearising your RGB spaces correctly before converting to Lab; every RGB space typically has a unique transfer function.

* Be well aware of the limitations of Lab, such as red turns yellow and blue turns purple hue nonlinearity etc.

* Be using an appropriate model for the intensities of light being compared. Lab, for example, is not well suited for anything beyond diffuse white reflectance values.

Colour should be able to handle all of the above points, and deliver great looking plots as well!