Purple is the colour of vital power or vital force. It corresponds to the main force of the vital. Also other colours can play, e.g. green and deep red as well as purple and there are other colours for the hostile vital forces.
A strong vital is one that is full of life-force, has ambition, courage, great energy, a force for action or for creation, a large expansive movement whether for generosity in giving or for possession and lead and domination, a power to fulfil and materialise many other forms of vital strength there are also. It is often difficult for such a vital to surrender itself because of this sense of its own powers but if it can do so, it becomes an admirable instrument for the Divine Work.
Even apart from yoga, in ordinary life, only those are considered to have full manhood or are likely to succeed in their life, their ideals or their undertakings who take in hand this restless vital, concentrate and control it and subject it to discipline it, compelling it do not what it wants but what the reason or the will sees to be right or desirable. In yoga one uses the inner will and compels the vital to submit itself to tapasya so that it may become calm, strong, obedient -- or else one calls down the calm from above obliging the vital the vital to renounce desire and become quiet and receptive.