The exploration of colour is one of the most exciting challenges open to the artist, and we should all be agreed that colour vision is a most exciting stimulus to human emotion. Simply put, the human eye and brain responds to electromagnetic waves within what we know as the visible spectrum. Each colour having its own specific frequency of vibration to which the eye is sensitive. Therefore the colour characteristics of a pigment depend on its ability to reflect and refract light. In otherwords, when white light falls on Cadmium Red for instance a majority of the green and blue/violet frequencies are absorbed and much of the red is reflected from the surface. The colours we see of any object then are those that are not absorbed.
Evidence also suggests that our colour vision is subjective. For instance a grey shape will appear lighter against a black background than when placed against a white background. Similarly, juxtaposed dots of cadmium red and green when brought to a common tone by mixing with white will appear yellow when viewed at a distance. Whereas the mixing of these two colours on the palette will only result in a dull muddy colour. The pointillist technique and impressionists used this knowledge to great effect.
The traditional colour circle has served artists well in the practical application of mixing pigments. Red, Yellow and Blue are regarded as the Primaries from which the secondaries (orange, green and violet) can be mixed. However it should be noted that the secondaries thus obtained are often muddied and lack intensity due to the impurities of the pigments. So if your requirement is a vivid secondary, then you should use the closest purest pigment available and avoid mixing. Each Primary colour has a secondary pair. These are known as "complimentary colours" and they are a constant. Red-green, Blue-orange, Yellow-violet. You will notice that each pairing is gained by mixing two Primaries and pairing it with the third. For instance Blue-orange (Red mixed with Yellow). It sounds simple but in practise is not due to the impurities of pigments and the vast range of Reds and Blues etc can lead to unexpected results. Providing the artist understands the importance of complimentary colours they can then start to wrestle with more important aspects of colour design. For instance, the addition of an intense area of colour may suddenly throw all the earlier work into hues of grey as the eye seeks to find that subjective juxtaposition of contrast and colour.
For example: a predominantly red painting may not appear so red in a red room. Understanding the physical properties of colour and the human response can only help you in your choice of pigments.