I doubt there are any 'colourless' people as you infer, Grace, for everyone has within them the 'spark of life' which is the essence of what we all are. However, the less thick the egotistical shell (the less the self-image) the more we can be our natural self. It is the Natural Self that shines through (that is natural to ourselves in our own estimation) that is the deciding factor as to whether we are confident, outgoing, and uninhibited.
(This, of course, is only me opinion. It doesn't means it's right. It only means I think it could be right)
Charisma comes from the energy which radiates outward from us. We give out what we have inside always (unless we are a saint) through the conditioning of our minds, our beliefs. If we believe we a 'great' (not in an egotistical way) and that we have great energy and power which we can use to lead, persuade and inspire others, then we have it.
The more we get to know our true self, our real self, the more of charismatic we become. Confidence springs from self-esteem, real self-esteem, that comes without fear from the self which lies within us all.