Ok thanks a lot.
I have evaluated this topic further, just in case!
If I am using Navigation(Tab Mode on all Plattforms), I have to change the color via:
Theme.navigationTabBar.titleColor
Theme.navigationTabBar.titleOffColor
this affects both title and icon.
After replacing the Navigation Element with the TabControl,
“Theme.navigationTabBar.titleColor” and “Theme.navigationTabBar.titleOffColor”
only affects the color of the icon.
To change the color of the title I had to set the property “titleColor” and “titleOffColor” inside the TabControl