A thought, then, as stated, becomes for the time a kind of living creature; the thought-force is the soul, the elemental essence of the body. These thought-forms are called elementals, or sometimes artificial elementals.
The principles underlying the production of all thought-forms are:
 Quality of thought determines colour
 Nature of thought determines form
 Definiteness of thought determines clearness of outline.
Thought-forms may be of infinite variety, both as to colour and shape. With the various colours and their significance the student will now be familiar, as they are in agreement with those existing in the astral and mental bodies as described in The Astral Body and also in an earlier chapter of this book. Thus for example, affection produces a glowing rose-colour; a wish to heal, a lovely silvery-white; a mental effort to steady and strengthen the mind, a beautiful flashing golden-yellow. Yellow in any of the vehicles always indicates intellect, but its shades vary much, and it may be complicated by the admixture of other colours.
Generally speaking, it has a deeper and duller tint if it is directed to the lower channels, more especially if the objects are selfish. In the astral or mental body of an average man of business it would show itself as yellow ochre, while pure intellect devoted to the study of philosophy or mathematics appears frequently as golden; this rises gradually to a beautiful clear and luminous primrose-yellow when a powerful intellect is employed absolutely unselfishly for the benefit of humanity.
Most yellow thought-forms are clearly outlined, a vague cloud of yellow being comparatively rare. It indicates intellectual pleasure such as appreciation of the result of ingenuity, or the delight in clever craftsmanship. A cloud of this nature betokens the entire absence of any personal emotion, for if that were present it would inevitably tinge the yellow with its appropriate colour. In many cases, thought -forms are merely revolving clouds of the colour appropriate to the idea which gave them birth.
The student will realise that, at the present stage of humanity, there is a vast preponderance of cloudy and irregularly-shaped thoughts, the product of ill-trained minds of the majority. It is among the rarest of phenomena to see clear and definite forms among the thousands that float about us. Where a thought is definite a form is created, and a clear-cut and often beautiful shape is assumed. Such shapes while of infinite variety, are often in some way typical of the kind of thought which they express.
***From Arthur E. Powell – The Mental Body