stock SetPlayerRainbowColor(playerid, index, maxPlayers)
{
   new
      color_Red,
      color_Green,
      color_Blue
   ;
   color_Red = floatround((0xFF * floatsin((index / (maxPlayers / 3.0)), radian)));
   color_Green = floatround(((0xFF / maxPlayers) * index));
   color_Blue = floatround(((0xFF / maxPlayers) * (maxPlayers - index)));
   
   return SetPlayerColor(playerid, ((color_Red * 0x1000000) + (color_Green * 0x10000) + (color_Blue * 0x100) + 0xFF));
}

public OnPlayerConnect(playerid)
{
   for(new i = GetMaxPlayers(), totalPlayers = GetOnlinePlayersCount(), index = totalPlayers; i > -1; --i)
   {
      if(IsPlayerConnected(i) && !IsPlayerNPC(i))
      {
         SetPlayerRainbowColor(i, index--, totalPlayers);
      }
   }
   return 1;
}

public OnPlayerDisconnect(playerid, reason)
{
   for(new i = GetMaxPlayers(), totalPlayers = GetOnlinePlayersCount() - 1, index = totalPlayers; i > -1; --i)
   {
      if(IsPlayerConnected(i) && !IsPlayerNPC(i) && (i != playerid))
      {
         SetPlayerRainbowColor(i, index--, totalPlayers);
      }
   }
   return 1;
}

stock GetOnlinePlayersCount()
{
   new
      totalPlayers
   ;
   for(new i, j = GetMaxPlayers(); i != j; ++i)
   {
      if(IsPlayerConnected(i) && !IsPlayerNPC(i))
      {
         ++totalPlayers;
      }
   }
   return totalPlayers;
}