“One segment visits these tunnels and has color footage taken at the time.”
Taken at which time? It’s highly unlikely that color film existed in WW1.
It actually did and that is what makes the series so good. The color adds a totally different dimension to the WW I films. They are scarce and the footage is in very short segments. It is not "colorized".
Like the World War II in color series that has voice over the film segments the WWI narrator describes what is seen and how those segments fit in to the overall narrative.
Here 's some info