The library can use color profiles to create calibrated colors.
Semi-transparent colors and PDF form XObjects can be used to achieve nice visual effects.
You can specify how semi-transparent objects will blend on a canvas using the set of predefined blend modes.
You can use XObjects to create watermarks, backgrounds, or repeatable items for pages in your documents.
Each XObject has its own canvas, so you can create XObjects as complex as regular PDF pages.