There is no per-event coloring currently in Horde 5. Categories were 
replaced by tags. There is already an open enhancement request about 
adding colored event "dots" to the event bubbles for the event tags' 
color.
Already possible again in Kronolith 4.2
You can absolutely define multiple calendars in Kronolith. In Horde 
5.1/Kronolith 4.1 and prior, all events are "multiplexed" into a 
single calendar to sync. Starting in 5.2/4.2 it will be possible to 
truly sync multiple sources, but not every client supports it.
See the many mailing list and tickets that already discuss this.
That's a blatant generalization. If it's THAT important for you than 
consider sponsoring the feature to be developed.
Again, see the various mailing list discussions and tickets. The 
suggestion of just adding the "first" tag is not feasible for a 
variety of reasons.

In H3 the calendar shows a fine overview with a color pro category and 
the event time from .. to in the monthly overview. In Horde 5.1.3 the 
calendar overview is confused, because the event color and the event 
time are missing.

In Horde 5.1.3 I have to write in the subject the event time. So I see 
at least the start time in the monthly overview.

The definition of several calendars is impossible, because the sync 
with Android supports only one calendar. Is'nt it?

To work fine with the calendar again it's necessary:

- to show the defined tag-color in monthly/weekly/daily overview
   * has a event several tags, the color of the first tag is winning
- to show the event start-end-time in the monthly overview.

Otherwise the calendar is unfortunately a step backwards.

For example:
The google calendar shows the start time and an color point for every entry.
In Outlook I can defined several categories for an event. Outlook 
shows the color of first defined category.