Color Section Opacity

Hey,

I tried this Quick-CSS-code to make opacity of a color section to 0.7

#eleven { background-color: rgba(0,0,0,0.5); }
#main { background-color: transparent; }

(i put the #eleven also in the devolopment field of the color section.)

the color section that i want to make transparent, is between two other color sections (they have a fixed background picture).

But the result is: the color section getting a GREY (#7F7F7F) instead of beeing transparent.

Do you know a solution?
Thanks in advance