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
Select the tracks whose color you wish to change. Context-click on the track header of one of them. From the context menu, select Color and pick a hue to your taste in the color dialog. Every selected track will be re-colored.
Note that if you are only changing one track, context-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.
You can also explicitly change the group color by context-clicking on the group tab in the Mixer, selecting Edit Group… and then clicking on the Color selector in that dialog that is displayed.