I can see how that would be useful. There isn't a direct way to do that currently. I will look into that as a feature for a future release.
In the meantime, you could try this. Not sure if it will work for your data or if the current color coding is important.
1. Create a new continuous column and fill the entire column with the value 1.
2. Place the new column in the color role.
3. Right click on the color role and select Summary Statistic -> % of Total
4. In the properties pane, enable both the color and size labels.
See the attached screenshot. Hope this helps.