Bug 381496

I see. The problem is: in korganizer3 the color behavior was exactly the other way round: 
frame of the event = color of the resource
body of the event = category color.

Not arguing which way is better, I simply state that this is a change. An irritating change.

Furthermore, having only one event on a day without an associated time eliminates the frame. No frame - no resource color. 

Even stranger, starting korganizer3 in the current KDE4 environment shows the very same "inverted" color scheme. 

You want "Configure->Views->Colours used in agenda view" to change the item colouring policy back to the old style "Category inside, calendar outside".  This was backported to 3.5 branch as well.  I'm asking upstream why the behaviour change was made and if I may revert it.

Alternatively, calendars are automatically assigned colours, but categories 
are not.  Would ResourceOnly be a better default in KDE 4.1, to avoid 
information-free and usually colour-clashing green frames on items.  If the user sets category colours he/she can switch to a different policy.

Reverted for KDE 3.5, and I won't fix it for kde 4, the change is intentional to remove the amount of information-free colour but keep the frame so that people with category colours defined can see them.

Seems to be a new (to me, at least) configuration option:
Settings -> Views -> Agenda View -> first drop down list: Category inside, Calendar outside

This is perfectly fine, whatever the default may be.