IMO we should not be using action icons in these larger sizes in the first place--at least not in apps. The places where we are doing this should be corrected in the apps themselves.

For example Okular's sidebar category chooser is something I'm actually working on removing and transforming into a more standard combobox-based category chooser UI.

In the Places panel we should be using places icons for entries so that they have pretty large sizes.

Plasma uses large >22px monochrome icons in a lot of places, so maybe we should concentrate on that. I think they can look acceptable at up to 32px size, maybe with bigger line weights. At larger sizes, it's probably not appropriate to use a monochrome icon, which suggests that if only a monochrome icon is available, it should be 32px in size as a maximum, and if more space is needed, we should maybe enclose it in a visible button with larger bounds rather than making it a toolbutton.

In T11526#198388, @ngraham wrote:

IMO we should not be using action icons in these larger sizes in the first place--at least not in apps. The places where we are doing this should be corrected in the apps themselves.

For example Okular's sidebar category chooser is something I'm actually working on removing and transforming into a more standard combobox-based category chooser UI.

In the Places panel we should be using places icons for entries so that they have pretty large sizes.

Plasma uses large >22px monochrome icons in a lot of places, so maybe we should concentrate on that. I think they can look acceptable at up to 32px size, maybe with bigger line weights. At larger sizes, it's probably not appropriate to use a monochrome icon, which suggests that if only a monochrome icon is available, it should be 32px in size as a maximum, and if more space is needed, we should maybe enclose it in a visible button with larger bounds rather than making it a toolbutton.

I'm not too concerned about Plasma since it's still fine to put the 32px monochrome icons into the desktop theme.

What about 3rd party apps that use action icons in the sidebar because the icons are commonly supported by other icon themes?

What should I do about Plasma Browser Integration's need for large document-share and document-share-error icons in desktop notifications (https://bugs.kde.org/show_bug.cgi?id=410840)? Notification icons are normally 48px.

I guess we can create colorful versions of action items on a need-to-have basis, as long as we commit to not using them for actual actions in buttons etc. for our own apps.

In T11526#198734, @ngraham wrote:

I guess we can create colorful versions of action items on a need-to-have basis, as long as we commit to not using them for actual actions in buttons etc. for our own apps.

Agreed