Some MOnsters are Purple outlined and some orange

[Schiggy]

I need Heros Streamstone do both drop it ?

[VindooZero]

Purple outline mean Tempered (more difficult that normal) and orange means Arch Tempered (the hardest version of a monster) and they indeed have a change to drop it. Investigations were the rewards are purple boxes have a change to give streamstones too. Also quests like Throne Breakers and Tempered Djo could give you some.

[Pocahawtness]

Yes, both drop them, but they are rare. I mean I did hundreds of hours of the game before I even got one Hero Streamstone.

[Katitoff]

Purple are tempered monsters.
Orange are arch tempered monsters and elder dragons only.

Purple ones drop them sometimes.
Orange ones almost always.