It is crucial to define what your logo/logotype will look like
when appearing in full colour, spot colour, greyscale, and black-and-white.
The last two are extremely important if you are considering having
any work printed on newsprint (relatively low resolution, can
tend to look grainy if you're not careful).
These are usually defined by Pantone colour (PMS), RGB, and CMYK
colour systems. Different systems have different applications
and uses, so it is crucial that you define your colours in all
of these colour systems.