Since the latest release it is no longer possible to change panel launcher icons to something different to the main theme selected. You are still able to open the properties dialog for each icon and navigate through the selection process but then the final 'OK' setting to activate the change is greyed out.

Please report this to https://bugs.kde.org, the forum is the wrong place for bug reports.

Also specifying what exactly you mean by "latest release" is needed, as this can differ widely in distributions. System Settings -> System Administration -> System Information is the easiest way to get this info