practise - theory
In theory you know the bubble should be white in practise you use the pipette tool to be sure.
In the colour test you see ten green colours which you have to order, in practise you have 39 tones making up a sfx and you have to decide what is the best way to replicate them. While having a good colour IQ helps (often boosted by a good screen) it is essentail to have experience and a routine with the actual work and be proficient maybe even have a routine in the typical cleaning steps (like cleaning white bubbles).
I think experience is underestimated a lot, while some people learn faster and some need longer it always boils down to the time you put into it and the experience you have. Lets be frank cleaners don't need to be artists, they just need to know the basics to replicate what others already created.
Being a real artist never hurts