irssi/docs/FORMATS

                           text            text            background
   ---------------------------------------------------------------------
   %k      %K      %0      black           bold black      black
   %r      %R      %1      red             bold red        red
   %g      %G      %2      green           bold green      green
   %y      %Y      %3      yellow          bold yellow     yellow
   %b      %B      %4      blue            bold blue       blue
   %m      %M      %5      magenta         bold magenta    magenta
   %p      %P              magenta (think: purple)
   %c      %C      %6      cyan            bold cyan       cyan
   %w      %W      %7      white           bold white      white
   %n                      All colors turned off
   %U                      Underline on/off
   %8                      Reverse on/off
   %9      %_              Bold on/off
   %:                      Insert newline
   %%                      A single %

parameter handling:

$[30]1    prints parameter 1 cut/padded to 30 chars
$[!30]1   prints parameter 1 padded to min. 30 chars
$[-30]1   prints parameter 1 right aligned
$[30?]1   prints parameter 1 padded with '?' characters
$[30.0]1  prints parameter 1 padded with '0' characters
%|        marks the indentation position.