templates should be grey, enabled hosts should be blue and disabled hosts should be red. is that not the case for you ?

Sorry, just noticed, that I was not clear enough by saying 'CONFIGURATION OF HOST GROUPS'.

What I'm talking about are the selection lists which list hosts and templates in the same context.
For example the list for the selection of 'host' members in the configuration of a host group, or the selection of 'host/template' members in the configuration of a template.

Marc is absolutely right, voted !

Another way to achieve distinction could be to provide separate selection lists for each of them.

This would make selection of large ranges a little bit more comfortable.
E.g. with just a visual differentiation it might be necessary to review the whole selection, if one wants to be sure that only hosts are selected.

Edit:
But this wouldn't help for host/template selection in pull-down menus, e.g. in window for selecting an item for trigger creation.

Yet another idea is to group them by type.

ah, i see. grouping might work - see interfaces in item properties

Yes, that's a good idea!

It's a bit tricky with the current implementation of tween object

Patch follows - nevertheless without t(w)een object support though

The attached patches:
ZBXNEXT-1820-2.0.10.patch
ZBXNEXT-18202.0.10ZBXNEXT-370.patch (proper patch when ZBXNEXT-370 patch is applied)

modify behavior of getHostsCB().

When there gets no template listed in a (host) combo-box, then the behavior is the same as without the patch - lists hosts without any option group.
But if there will be at least one template listed in a (host) combo-box, then hosts and templates are grouped as seen in the screen-shots.
A host option group will not be displayed when only templates are listed.

Attached patch:
ZBXNEXT-1820-2.0.10-tweenboxes.patch

adds tweenbox related modifications.

It does not include combobox related modifications from ZBXNEXT-1820-2.0.10.patch

The patch separates hosts and templates in dedicated tweenboxes. Some labels are changed too.

A proper patch compatible with ZBXNEXT-370 can be provided on demand.

This patch is not tested very well yet!

in some places grouping would be ok, in others separate lists would be better - for example, "copy to" page - ZBX-7672

The problem with this solution is that it still treats hosts and templates as similar entities, when in reality they are completely different things. Ideally, they shouldn't appear together in a single list at all.

2.2 version from Marc, only implementing the dropdown menu part

Zabbix UI has undergone many changes. There are no more dropdowns (except maybe a few places) and starting from 6.2+ templates and hosts now are separated in template groups and host groups. The labels now clearly indicate what user is browsing and selecting. Whether it's a host or template. There should no longer be any confusion which is which (some bugs still might exist).
Closing the issue.
If you feel like there is something else to be done, feel free to reopen it.