Answer
In Colonia’s historic quarter, there is a network of small museums; around eight are often mentioned, including exhibits on colonial history, everyday life, archaeology, and communications. However, the number of galleries open to visitors varies: an individual museum may be closed for restoration, have reduced hours, or be temporarily excluded from the combined ticket.
Before your trip, check the official list and opening hours. If you have only one day, you do not need to visit everything: choose two or three exhibits near Plaza Mayor, and leave the rest of your time for Calle de los Suspiros, the ruins, the basilica, the lighthouse, and the waterfront.
If museums are your main goal, arrive in the morning and do not schedule your last visit just before the ferry. Small venues may close for a break, and the ticket office for the combined ticket and the museum exhibits that are open do not always follow the same schedule.