Hi Uwe Junker, welcome!
Good question, basically the answer is "yes" but also "it depends..."
Do you want all the tables on your site (on the wiki pages for instance) to have the same colours? If so the way to do it is with CSS - here is an intro to table borders styling https://www.w3schools.com/Css/css_table.asp and here is how to add custom css to your tiki.
If you want each table to have different colours on each page then you probably will need to use WYISWYG in HTML mode, you should be able to find some pointers to get you started here
You may find it helpful to join us on Gitter chat via https://gitter.im/tiki-org/community if you have more questions.
Hope that helps, welcome again!
Thanks for your quick replay and your welcome ;-)
I will try this.