Okay, finally got it!

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Assign %bgcolor.<channel> the background color and it will (or should) change the background. If you'd like a popup to help, here ya go:

Code:
on 1:JOIN:#: {
  if ($nick == $me) &amp;&amp; (%bgcolor. [ $+ [ $chan ] ] != $null) /background -f bg\ $+ %bgcolor. [ $+ [ $chan ] ] $+ .bmp
}

alias bgcolor {
  if ($dialog(bgcolor)) /dialog -vie bgcolor bgcolor
  else /dialog -m bgcolor bgcolor
}

dialog bgcolor {
  title "Background Color"
  size -1 -1 120 25
  option dbu
  box "Assign Background Color (&lt;chan&gt;)", 1, 0 0 120 25
  combo 10, 5 10 60 50, size drop
  button "Set", 20, 65 10 25 10, okay
  button "Cancel", 30, 90 10 25 10, cancel
}

on 1:DIALOG:bgcolor:INIT:*: {
  /set %bgchan $active
  /did -ra $dname 1 $replace($did(1), &lt;chan&gt;, %bgchan)

  ; set colors
  /did -a $dname 10 0 - White
  /did -a $dname 10 1 - Black
  /did -a $dname 10 2 - Dark Blue
  /did -a $dname 10 3 - Dark Green
  /did -a $dname 10 4 - Red
  /did -a $dname 10 5 - Dark Red
  /did -a $dname 10 6 - Violet
  /did -a $dname 10 7 - Orange
  /did -a $dname 10 8 - Yellow
  /did -a $dname 10 9 - Light Green
  /did -a $dname 10 10 - Cyan
  /did -a $dname 10 11 - Teal
  /did -a $dname 10 12 - Blue
  /did -a $dname 10 13 - Pink
  /did -a $dname 10 14 - Dark Gray
  /did -a $dname 10 15 - Gray
  if (%bgcolor. [ $+ [ %bgchan ] ] != $null) /did -c $dname 10 $calc(%bgcolor. [ $+ [ %bgchan ] ] + 1)
  else /did -c $dname 10 1
}

on 1:DIALOG:bgcolor:SCLICK:20: {
  /set %bgcolor. [ $+ [ %bgchan ] ] $calc($did(10).sel - 1)
  /background -f %bgchan bg\ $+ %bgcolor. [ $+ [ $chan ] ] $+ .bmp
  /unset %bgchan
}

on 1:DIALOG:bgcolor:CLOSE: {
  /unset %bgchan
}

menu channel {
  Change Background Color: /bgcolor
}


Also, here is the zip of all the colors. Make a folder in your mirc directory called "bg", and place 0.bmp~15.bmp in there. The script will do the rest. >:D

-KingTomato