How do I change the color of the pane dividing lines in tmux?

You want pane-active-border-style and pane-border-style:

See the entry in the man page:

pane-active-border-style style
Set the pane border style for the currently active pane. For how to specify style, see the message-command-style option. Attributes are ignored.

pane-border-style style
Set the pane border style for pane as aside from the active pane. For how to specify style, see the message-command-style option. Attributes are ignored.

So, in your ~/.tmux.conf you could specify colours like so:

# border colours
set -g pane-border-style fg=magenta
set -g pane-active-border-style "bg=default fg=magenta"

Note, I use tmux 1.9a, and I find I get more consistent behaviour using:

set -g pane-border-fg magenta
set -g pane-active-border-fg green
set -g pane-active-border-bg default

As of tmux version 2.9 @jasonwryan 's implementation is reduced to two lines:

set -g pane-active-border-style bg=default,fg=magenta
set -g pane-border-style fg=green

Note lack of spacing between bg and fg declarations

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