Color is the most basic element of a painting. Every color has three aspects to it: hue or name, value or tone, and chroma or saturation. If you're trying to mix a color on your palette to accurately match a color in a subject, you need to consider all of these to get the "color right".
A color in a painting isn't simply blue, red, yellow, etc, but a very specific blue or red as every paint color is made from a specific pigment(or mixture of pigments). Learning the characteristic of different pigments is crucial to mastering color in painting. Ultramarine blue, Prussian blue, and cerulean blue are distinctly different blue pigments, for instance.
The label on a tube of paint will tell you what pigment is used to make the color. Each pigment has a Color Index Number which makes it possible to identify when the same pigment is being sold under different generic color names.
