Typically we are using reflection mode where we measure light reflected from the object. Pigments, colorants, and dyes on the sample surface absorb a portion of the light used to enlighten.  Today we have two basic types of instruments capable to measure the color of the sample:

• colorimeters - capable to return colorimetrical coordinates
• spectrophotometers - where we get information on reflection split into wavelength bins

To learn more about the differences read the document: Spectrophotometer vs Colorimeter which answers the question:
How do the instruments see color? 

Another document:  Maturity of Color Definition explains the steps of advancement levels.

Lab, LCH, RGB, XYZ, CMYK,  ...

There are a lot of color spaces used for the color description. Most users used to think that if they specify color in Lab or RGB that is a precise color definition, while it is not if we don't specify which Lab, RGB or CMYK space we are thinking about.

 CIE Lab D50/2° ≠  CIE Lab D65/10°

CIE Lab ≠ Hunter Lab

sRGB ≠ Adobe RGB

CRPC2 CMYK ≠ GRACoL CMYK

It is also very important not to mix:

• Color Specifications
• Measurement Geometry
• Measurements Conditions
• Color difference formulas

There are a lot of very good sources of general color science knowledge - we can recommend some of them!