How to change "activity in window" color in byobu

Given the following byobu (with tmux backend) status bar, how can the window activity notification colors be adjusted? Since they are currently the same as the "active" window, it is difficult to tell the difference.

Sure! You can set it like this, in your ~/.byobu/.tmux.conf:

set -g window-status-activity-bg red

For a complete list of color options, refer to tmux.1 and specifically its OPTIONS section

Full disclosure: I am the author and maintainer of Byobu.