Translucence is often misused when talking about transparency. Transparency is generally associated with clear. The more correct term for how well you can "see" through something is opacity.

Where you may be confused is that, in order to have 20% opacity in the #FFFFFF, the 33 is added at the beginning rather than at the end AND the result should be eight (8) versus six (6) characters. Had you entered #33FFFFFF, it would have worked IF your browser supported it.

It is the old method and doesn't give the non-compatible browser any alternative. Another reason for the dual calls in the css.

Mentioned it in an earlier thread but, http://tinyurl.com/y929h5f6 is a good read. Especially near the end of the article. Click on View Demo just before the Conclusion.