1) "The juvenile mangosteen fruit, which does not require fertilization to form (see agamospermy), first appears as pale green or almost white in the shade of the canopy. As the fruit enlarges over the next two to three months, the exocarp color deepens to darker green. During this period, the fruit increases in size until its exocarp is 6–8 centimeters in outside diameter, remaining hard until a final, abrupt ripening stage. The subsurface chemistry of the mangosteen exocarp comprises an array of polyphenolic acids including xanthones and tannins that assure astringency to discourage infestation by insects, fungi, plant viruses, bacteria and animal predation while the fruit is immature. Color changes and softening of the exocarp are natural processes of ripening that indicates the fruit can be eaten and the seeds are finished developing." Source and further information: http://en.wikipedia.org/wiki/Mangosteen 2) "The rind of the fruit is deep reddish purple when ripe." Source and further information: http://www.squidoo.com/mangosteen-fruit 3) "Selection & Storage: The mangosteen is ripe when its outer skin is slightly soft to the touch and its purple color is fully developed. Ripe mangosteens keep well for 3 to 4 weeks in storage at 40 to 55 F (4.44-12.78 C). Longer periods cause the outer skin to toughen and the rind to become rubbery; later, the rind hardens and becomes difficult to open and the flesh turns dry." Source and further information: http://www.blueproduce.com/mangosteen.aspx

The skin of the mangosteen will be purple when it is ripe. Just take a look at the picture I attached below.