Transparent colors

(Posting via Yahoo's website because my emails never seem to make it
.. Yahoo's groups suck :-/)

Is there any way to create a 'transparent' object?

I'd like to lay down a rect() in yellow that 'highlighted' a part of
the page. So the yellow would need to be transparent. Something like
'multiply' in Photoshop or Illustrator.

Cheers!
Rick Measham