DS firmware versions
Nintendo DS Lite showing the firmware version by way of the Pictochat Test
The firmware version of a DS can be confirmed using the Pictochat Test,   also called TakeMeOut: 
Insert a DS cartridge into the DS.
Turn on the DS, open Pictochat and enter room A.
Remove the cartridge.
Both screens should now be filled with a solid color.
Additionally, this color is also shown for a brief second before the DS enters Sleep Mode when the lid is closed.
Version 1: The DS will freeze
Version 2: Grayish Blue
Version 3: Dark Green
Version 4: Yellow
Version 5: Magenta
Version 6: Blue
Version 7?: Purple
iQue: Dark Green (special chinese ds)
DS Lite: Purple
FlashMe: Dark Green (when you flashed you're ds with costum firmware)