New tracks in Ardour are assigned a random color from a pastel color palette, so they should never end up being particularly bright or particularly dark.
Changing the color of specific tracks
Changing the color of a track is done by selecting the track(s) and right clicking on the track header of one of them then from the context menu, selecting Color and picking a hue in the color dialog. Every selected track will be re-colored.
If only one track is changed, right clicking on that track's header will be enough to select it, saving the extra mouse click.
Changing the color of all tracks in a group
Tracks that belong to a track/bus group can share a common color by enabling the Color option for the group. With this enabled, any color change will be propagated to all group members.
The group color can also be explicitly changed by context-clicking on the group tab in the Mixer, selecting Edit Group… and then clicking on the Color selector in the displayed dialog.