If you would like to use the Freie Universität green color as a highlighting color (please use sparingly), then we can recommend this trick.

Create a new object on one of your slides (e.g., a square) and fill it with the color you would like to use for highlighting. 
After you have done this, click on the object that you have created and go to the “Shape Format” tab. Select “Shape Fill” and click on the “Eyedropper” in the pop-up menu. With the eyedropper, select the color that was initially applied to your object (square). You can then delete the object (square).
Now select the text/word that you would like to highlight. The color you picked up with the eyedropper should appear in the section “Recent Colors” below the standard colors in the drop-down menu for text highlighting color. Now you can select and use it as a text highlighting color.

Please note that the “Recent Colors” section may be deleted automatically when you close PowerPoint. This does not usually happen when you use “Recent Colors” and save your document. If it does happen, you must repeat the steps listed above when you reopen your document.