It is possible to output the file from OmniGraffle in a texteditor.
If you do not compress the file, the file will be a XML-file, which can be read by any ext editor.
What I did was searching for some color information in combination with some font information. I found this in my file:
\f0\b\fs24 \cf0 \cb2 zoeken}</string>
I replaced all lines that look like this:
I figured the first three values are defining the used color definition (rgb, hsl, cmyk) and the second one might give the used color.
Iím not sure, but I succeeded in removing the background color from the text. Maybe removing the line completely works as well, Iím not sure. Maybe this also removes the color from the text itself, Iím not sure. More research might be necessary, but for me this did the trick. :-)
I hope this might be of some use to others as well.