Hi Ulrich

Thanks for reaching out on the Lucidchart Community! There are a few ways that you can change the colors for shapes on your doc. You can choose one of our pre-created themes on the right panel under theme and choose colors that way. 

The other option if you want a color other than those provided under themes is to select a "Fill Color" from the top bar.

If these options do not work and  "Fill Color" is grayed out it is possible that it isn't one of the shapes for which the color can be changed. Not all custom shapes support the color being changed. If you provide a screenshot of the custom shape I can provide further context of whether it falls under this category or not. 

Hope this helps!