Demostración de la mecánica: https://imgur.com/UNaAm9s

Scripts:

Se define en PBTerrains los tags isLuz? y isSombra?
Se define F como input en Controls
Se definen las formas en MultipleForms

---En Scene Map---
   #Cambio de forma 802
    if Input.trigger?(Input::F) && !pbMapInterpreterRunning? && $game_switches[101]
          for i in $Trainer.party
            if i.species==802
              if  $game_variables[101]==1 #si es luz, pasa a sombra
               if PBTerrain.isLuz?(pbGetTerrainTag)
                 Kernel.pbMessage(_INTL("¡No puedes transformarte aquí!"))
                 break
               end
               i.resetMovesOddish(0)
               i.form=0
               $Trainer.outfit=0
               $game_variables[101]=0
             else #si es sombra, pasa a luz
                if PBTerrain.isSombra?(pbGetTerrainTag)
                   Kernel.pbMessage(_INTL("¡No puedes transformarte aquí!"))
                   break
                end
                i.resetMovesOddish(1)
                i.form=1
                $Trainer.outfit=1
                $game_variables[101]=1
              end
            $game_map.refresh
            pbSEPlay("expfull")
            break
          end
         end
        end


----En Game_Map----
  #Sombra y luz comprobaciones
      elsif PBTerrain.isLuz?(@terrain_tags[tile_id]) && $globalluz==0
        Kernel.pbStartOver
      elsif PBTerrain.isSombra?(@terrain_tags[tile_id]) && $globalsombra==0
        Kernel.pbStartOver
      elsif PBTerrain.isLuz?(@terrain_tags[tile_id]) && $game_variables[101]==0
          return false
      elsif PBTerrain.isSombra?(@terrain_tags[tile_id]) && $game_variables[101]==1
          return false

------En PBField_Field, bajo field_movement------
$globalluz=250
$globalsombra=250

Events.onStepTaken+=proc{
  if PBTerrain.isLuz?(pbGetTerrainTag)
    $globalluz-=5
  elsif PBTerrain.isSombra?(pbGetTerrainTag)
    $globalsombra-=5
  end
}

--------En el script de Simple Hud de FL, en Coded Blue Bar-----------
 if USEBAR
      bar=IconSprite.new(0,yposition,@viewport1)
      bar.bitmap=Bitmap.new(Graphics.width,Graphics.height)

        colors=[
        Color.new(255,255,255),Color.new(160,160,160),     # Luz
        Color.new(0,0,0),Color.new(160,160,160),      # Sombra
     ]
    if $game_switches[101]==true
    #Luz
    bar.bitmap.fill_rect(Rect.new(10,40,250*10/20,6),colors[1])
    bar.bitmap.fill_rect(Rect.new(10,40,$globalluz*10/20,6),colors[0])
    end
    #Sombra
    bar.bitmap.fill_rect(Rect.new(10,50,250*10/20,6),colors[3])
    bar.bitmap.fill_rect(Rect.new(10,50,$globalsombra*10/20,6),colors[2])

    bar.z = 1000
    @hud.push(bar)

 end