Tmux monitor activity change highlight color

You might be running into the fact that the default window-status-activity-attr is reverse. This ends up switching the foreground and background colors (i.e. you see red letters when you specified the -bg to be red).

Try adding this:

set-window-option -g window-status-activity-attr none

Or, try adjusting or switching your window-status-activity-fg and window-status-activity-bg settings.