The most important aspect of the title is the way that it draws attention to the importance of colours in the novel. If you look very carefully, it is clear that the novel uses colours as a kind of motif to comment on the extent to which various characters are free or not. Thus brighter colours are used to indicate liberation, and colour is utilised to highlight resurrections in various characters during the novel. Consider for example when Kate and Celie go looking for a new dress, but the only colours that are available are neutral, drab colours such as brown and dark blue. However, Sofia and Celie use a bright yellow material from a dress of Shug's to form a quilt. In particular, you might want to examine how the colour purple is explicitly linked to the religious epiphany of Celie when she changes her attitude towards God. Consider the following quote:
Well, us talk and talk about God, but I’m still adrift. Trying to chase that old white man out of my head. I been so busy thinking bout him I never truly notice nothing God make. Not a blade of corn (how it do that?) not the color purple (where it come from?)...
This comes from the seventy-third letter of the novel, and focuses on Shug's conversation with Celie when she helps her to change her understanding of God, moving away from imagining God to be a white male oppressor and towards conceiving of God as an entity with whom Celie can identify more easily. Shug therefore tries to encourage Celie to see God in all things around them rather than imagining God as being white and male. Reimagining God in this way leads Celie to be able to wonder at the simple miracles of life, of which the colour purple is one. The colour purple therefore draws attention to the key theme of reimagining our oppressors to help us face them and cope with our situations of hardship.