Code I

property pMember -- Bitmap member used for canvas
property pRect   -- rect of altered area
property pBefore -- image of altered rect before
--                  alteration
property pAfter  -- image of altered rect after alteration

on new(me, aDataList) ------------------------------------
  -- INPUT: <aDataList> must be a property list with the
  --         structure:
  --         [#member: <bitmap member>,
  --          #rect: <rect>,
  --          #before: <image>,
  --          #after: <image>]
  -- ACTION: Stores a pointer to the member whose image is
  --         altered, along with the rect of the dirtied
  --         area and the image of that rect before and
  --         after the change
  -- OUTPUT: Returns a pointer to this instance
  --------------------------------------------------------

  pMember = aDataList.member
  pRect   = aDataList.rect
  pBefore = aDataList.before
  pAfter  = aDataList.after

  return me
end new

Code II

property pColor
on Paint_SetBrushColor(me, aColor)
 if pColor = aColor then
  -- The color has not changed
  exit
 end if
 
 pColor = aColor
 sprite(me.spriteNum).color = pColor
end Paint_SetBrushColor

Code III

on Paint_SetBrushColor(me, aColor, isUndo)
  if pColor = aColor then
    exit
  end if
 
  -- Add your lines here
  if not isUndo then
  tUndoData = [:]
  tUndoData[#before] = pColor
  tUndoData[#after] = aColor
  UndoAction("Select Color Undo", tUndoData)
 end if
 
 pColor = aColor
  sprite(me.spriteNum).color = pColor
end Paint_ShowBrushColor

Code IV

property pBefore -- color before alteration
property pAfter  -- color after alteration

on new(me, aDataList) ------------------------------------
  -- INPUT: <aDataList> must be a property list with the
  --         structure:
  --         [#before: <color object>,
  --          #after: <color object>]
  -- ACTION: stores the color before and after the color
  --         change
  -- OUTPUT: a pointer to this instance
  --------------------------------------------------------
  
  pBefore = aDataList.before
  pAfter = aDataList.after
  return me
end new


on undo(me) ----------------------------------------------
  -- SENT BY the mDo() handler of the Undo Step instance
  --         that this instance is the ancestor of
  -- ACTION: restores the brush to its original color
  --------------------------------------------------------

  isUndo = TRUE
  sendAllSprites(#Paint_SetBrushColor, pBefore, isUndo)
end undo


on redo(me) ----------------------------------------------
  -- SENT BY the mDo() handler of the Undo Step instance
  --         that this instance is the ancestor of
  -- ACTION: re-adopts the altered brush color
  --------------------------------------------------------

  isUndo = TRUE
  sendAllSprites(#Paint_SetBrushColor, pAfter, isUndo)
end redo