I do not believe DROP can be used as an event in a dialog like this

on *:dialog:colormod:drop:991: {
You would have to use Sclick

on *:dialog:colormod:sclick:991:

Here is an example of a combo box I use

Code:
  [color:red]Menu channel,menubar,query,status {
  Nick:dialog -mo dcn dcn
}
dialog dcn {
  title "Nick"
  size -1 -1 162 84
  option dbu
  combo 4, 4 7 35 70, drop
  button "On/Off", 25, 4 70 35 10
  button "Cancel", 26, 123 70 35 10, cancel
  button "Update", 27, 40 70 35 10, ok
}

on *:dialog:dcn:*:*:{
  if ($devent == init) {
    if ($did == 0) { 
      dcnon4
    }
  }
  if ($devent == sclick) {
    if ($did == 4) { if ($did(4).seltext) { set %on.clr $did(4).seltext | /dcnon4 } }
  }
}


alias -l dcnon4 {
  did -a dcn 4 01 White | set %dcnon 0
  did -a dcn 4 02 Black | set %dcnon 1
  did -a dcn 4 03 Dk.Blue | set %dcnon 2
  did -a dcn 4 04 Green | set %dcnon 3
  did -a dcn 4 05 05 Red | set %dcnon 4
  did -a dcn 4 06 Maroon | set %dcnon 5
  did -a dcn 4 07 Purple | set %dcnon 6
  did -a dcn 4 08 Orange | set %dcnon 7
  did -a dcn 4 09 Yellow | set %dcnon 8
  did -a dcn 4 10 Lt.Green | set %dcnon 9
  did -a dcn 4 11 Teal | set %dcnon 10
  did -a dcn 4 12 Cyan | set %dcnon 11
  did -a dcn 4 13 Blue | set %dcnon 12
  did -a dcn 4 14 Pink | set %dcnon 13
  did -a dcn 4 15 Dk.Grey | set %dcnon 14
  did -a dcn 4 16 Lt.Grey | set %dcnon 15
  did -c dcn 4 %on.clr 
}
  [/color]