If you want to design transparent diagrams in Visio do the following:
- Right click on one diagram object
- Scrolldown to "Format" option & select "Fill"
- Once in the Fill section, move the transparency bar to the required level
- Click on "Apply" to view the change & if satisfied click on OK to quit
Note: This should be done using 2 or more object each one having a different colour.
As simple as 123.
Hope this answered your question.