Setting the colorbox opacity by hash parameter when initializing doesn't work the way like the documentation Show archive.org snapshot tells you.

  $(selector).colorbox({ ..., opacity: 0.5, ... }); // has no effect

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The opacity value of 0.5 will be overwritten by the inline style attribute style="opacity: 0.9" that colorbox sets.

To manually set the opacity you have to add the following css rule

  #cboxOverlay { opacity: 0.5 !important; }