As advancement continues it is gradually stirred into alertness by vibrations which reach it from the lower bodies.

This comes but slowly, because the activities of man in the earlier stages of his evolution are not of a character to obtain expression in matter so fine as that of the higher mental body; but when a man reaches the stage where he is capable either of abstract thought or of unselfish emotion the matter of the causal body is aroused into response.
When these rates of undulation are awakened within him they show themselves in his causal body as colours, so that instead of being a mere transparent bubble it gradually becomes a sphere filled with matter of the most lovely and delicate hues--an object beautiful beyond all conception.

It is found by experience that these colours are significant.

The vibration which denotes the power of unselfish affection shows itself as a pale rose-colour; that which indicates high intellectual power is yellow; that which expresses sympathy is green, while blue betokens devotional feeling, and a luminous lilac-blue typifies the higher spirituality.

The same scheme of colour-significance applies to the bodies which are built of denser matter, but as we approach the physical world the hues are in every case by comparison grosser--not only less delicate but also less living.
In the course of evolution in the lower worlds man often introduces into his vehicles qualities which are undesirable and entirely inappropriate for his life as an ego--such, for example, as pride, irritability, sensuality.
These, like the rest, are reducible to vibrations, but they are in all cases vibrations of the lower subdivisions of their respective worlds, and therefore they cannot reproduce themselves in the causal body, which is built exclusively of the matter of the three higher subdivisions of its world.