Purple stars or stars 6-10 give the hero acces to other heroes perks. To get to 6 stars is an extra 70 super shards. Super shards have a chance of dropping after a hero is at 5 stars, where the normal shards, as well as turning into rigs have a small chance of becoming super shards which are used to get stars 6-10. Once at 6 stars or above, any hero you have unlocked now has their perk available for your 6 star or above hero to have appear in the perk rotations. There is however a max of 1 other heroes perk per run, so once you pick another heroes perk you can’t get anymore.
Based on testing at 6 star there appears to be approximately an 8% chance that any given selection of 3 perks will conain another heroes perk.