Create a transparent color in PDF

I'd like to create a transparent color in PDF (directly, no external software). I know for example with x y z rg x y z RG I can set the rgb color values for stroking and non-strok开发者_JS百科ing operations. How do I create a transparent color? A link to the correct search terms in the pdf spec would be a great help.

Transparency in PDF is a property of objects, not colors (Wikipedia).

The Wapedia contains a description of the PDF file format. See Section 3.2.4. for "details" on transparency. Like all other resources on the web, they are mostly superficial.

I suggest to use a tool from Adobe to create the four forms of transparency and examine the result.