“Colour order system – arrangement of samples according to a set of principles for the ordering and denotation of their colour, usually according to defined scales.
NOTE – A colour order system is usually exemplified by a set of physical samples, sometimes known as a colour atlas. This facilitates the communication of colour but is not a prerequisite for defining a colour order system.”

an alternative definition is

“Colour (order) system – general term used for any effort to systematically arrange colour experiences.”

Among all the systems available, five stands out for historical reasons, knowledge and cognitive value: the Ostwald system, the Munsell system, the DIN‚ÄêColour Chart, the OSA‚ÄêUCS and the NCS. All five systems are outstanding for the rigour of their construction. In this topic we will cover 2 of these:
1. Munsell

2. NCS

At this point take note of the idea of a colour space. A simple definition is “Colour space – geometric representation of colour in space, usually of 3 dimensions.” A note worth taking is that here the word ‘space’ should not be considered as a mathematical space with its associated operations, excluding certain partial and particular cases, but as an interpretation graph and a three‐dimensional ordering of physical samples. The idea of a mathematical space exists where systems are important for measuring and communicating colour difference mathematically.

Some further reading on the Munsell System by following the link given. A short description of Munsell is given in this link.