Hop on hop off bus for red line route from cruise terminal

In Athens the only lines which are officially referenced by their color are the metro lines (green - red - blue)

So I'm not sure which bus company you're talking about and why you prefer one color over another. And I also don't know what place you're talking about when you say "transportation station"

There are plenty of Hop on-Hop off companies, and if they all take roughly the same route, the same color of bus can have several slightly different routes.

This route only follows the main avenues of Athens with regulated stops in a few places, which most of the time are not close to the major sites. The major sites are mostly in pedestrian areas and accessible only on foot.

I want to know how to get to the red line on the Hop on hop off tour bus company, from the cruise port. The
Hop on hop off bus company does not have a red line to the cruise port. it looks like I will have to transfer from another color line (using the Hop on hop off bus company) from the cruise port. If someone has done this, your information would be helpful.
I am not talking about the local bus line. Thanks in advance for your help.

This is what 'What's in Port' says about Pireaus port (that is the best website for pure factual information about any cruise port)-

Cruise ships dock at the New Passenger Terminal, one mile from the centre of Pireaus. The three terminals A (Miaoulos), B (Thermistocles) and C (Alkimos) are less than 1/2 km apart.

Athens HOHO tour starts at about 200 metres from the cruise terminal. Turn left from the cruise terminal building (lower level) and walk over the parking lot.

There is also a new bus service X80 every 30 minutes from 7am to 9.30pm to Acropolis and Syntagma- the stop and manned ticket booth is just outside Terminal B.

It is a 20 to 30 minute walk to Metro Line 1 (the green line). Change off the green line to the red line (Line 3) at Monastiraki.

There is also the Athens Pireaus electric railway (ISAP) from a different (close by) station in Pireaus-
https://www.stasy.gr/en/timetables/line-1/
That connects to the red metro line at Attiki, Omonia and Monastirak

Looking at City Sightseeing their HoHO service (their green line, not to be confused with the metro green line) stops at Cruise Terminals A and B- https://www.hop-on-hop-off-bus.com/athens/athens-hop-on-hop-off-tour_23374

City Sightseeing has several routes- transfer points between them (see the map on the website) and a day or 3 day ticket which covers all the lines.

So there are other options than taking the HoHo to a metro line.

Ideally you need to know which terminal- A, B and C your cruise is using, and the name of your hotel would also be useful.

To find out which terminal you need to ask your cruise line- there is a list on the Pireaus Port Authority website- but they use official names for the various berths so it doesn't really match up- https://www.olp.gr/en/services/cruise-terminal

see under Cruise Ships Arrivals- Download attachements- Cruise Schedule for 2024 Excel

I think I understand what "red line" you are talking about.
It is a commercial name for one of the many Hop on - Hop off bus companies.
We can see this "red line" which starts from Syntagma Square by clicking on map:

https://city-sightseeing.com/en/43/athens/53/hop-on-hop-off-athens.

You will need to go to Syntagma Square.
There is the X80 bus which operates from mid-May to mid-October.
Timetable below:

https://www.oasa.gr/en/telematics/?tl=lineDetails_1222_%CE%A780%20:%20PEIRAIAS%20-%20AKROPOLI%20-%20SYNTAGMA%20(EXPRESS)_67-99

Regarding the metro, the source of information cited by isn31c "What's in Port" is not up to date and does not take into account the extension of the Blue line which now goes to the port of Piraeus.
Anyway, the metro is too far from the cruise ports.

My best option: Take a taxi to Syntagma square.

In my opinion, 40€ per person for this route on this "red route" is really not worth it.
You can do the same route for less than €5 by public transport.

The Blue Line does not go to the port according to the Athens Metro themselves, Urban Rail and Google Maps- after Piraeus (same place as the Green Line) it continues one station to and terminates at Dimotiko Theatro. That is a bit closer to where some of the ferries leave from, but not materially closer to any of the three cruise terminals.

That was fact checked this morning.

The Blue Line does not go to the port according to the Athens Metro
themselves, Urban Rail and Google Maps- after Piraeus

That was fact checked this morning.

I don't know how you checked but the metro blue line stops at Piraeus station before Dimotiko Theatro. The metro station is exactly the same place as the green line station, but underground. This extension was put into service in October 2022
I have already taken it two times since 2022.

The urban rail station (Proastiakos) is located in another building behind the old green line station.

These metro stations are close to the ferry port but about a 30-minute walk from the cruise terminals.

There is no point in going to Dimotiko theatro station, it is hardly any closer and the station is less easy to locate.