The Color Purple could be said to have many themes and the actual color itself has some historical significance in different ways. For example it has been linked to royalty, and also to status. We believe that important Romans had their togas trimmed with this colour, so it may be a sign of respect - something that maybe had to be earned.
The color purple has also represented suffering, often used to describe blood or bruises. In Christianity, this may be associated with 'the blood of Christ' which the priest holds up to the heavens in a chalice. This of course, talks of the suffering, passion, torture and death of Christ the innocent victim. Think also of the saying 'the blood of the lamb.' Think about these images and then study the texst for themes of suffering, pain, disclosure and non-disclosure, mistreatment and physical harm, preferably with quotes and examples.
The color purple could also be said to be a sign of freedom, of a coming of age and a right to speak out as a free man or woman in the forum of human debate. It would also be helpful to read some of the author's own accounts of the writing and early life.