How to change the monochrome system tray icons to colorful icons?

The system tray has all the monochrome icons (except of Shutter). Some icons are confusing in high resolution desktop and small icons size. I feel they must be colorful to distinguish them from each other.

In the System Settings application > Icons, there are 4 themes Adwaita, Breeze, Breeze Dark and Oxygen. Applying any of them is not changing anything in the system tray. The Global Theme and Plasma Style in the System Settings application also has no effect.

How to change those icons?

I have Fedora 31 with KDE Plasma 5.18.5.

The icons are fetched from the plasma theme you use and if the plasma theme doesn't have some icons then they are fetched from the default theme. I guess the theme you used doesn't have icons. I'm also not sure there are themes with colorful system tray icons.