The official IWD website explains the history of the day:
"The day has occurred for well over a century, with the first IWD gathering in 1911. The day is not country, group or organization specific - and belongs to all groups collectively everywhere. It is a global day celebrating the social, economic, cultural and political achievements of women. The day also marks a call to action for accelerating gender parity."
According to the IWD website, purple is a colour for symbolising women, signifying justice and dignity. The colour, along with green and white, was historically associated with The Women's Social and Political Union (WSPU); a women-only political movement and leading militant organisation campaigning for women's suffrage in the United Kingdom from 1903 to 1917.
The above image is taken from the V&A: It depicts the weekly newspaper of The Women’s Social and Political Union.