If you stop at the 10-12 second mark of part 2 opening, you can see hermit purple whilst joseph is split in three
This was also confirmed by Araki in his interview :
"Hamon is life energy that travels through the entire body through a special breathing method. Therefore, a concrete visualization of it would be some sort of wire wrapped around the entire body, like a vine. If Jotaro's group time traveled to the world of Part 2, they would probably be able to see "Hermit Purple" wrapped around Joseph just like in Part 3."
I imagine it's similar to ghosts in that Ripple is not a Stand, but it takes on a visually different form to Stand Users due to their higher "supernatural presence". When Joseph's Stand unlocked, this is the form it took.
That said, the concept of Stands was likely developed after part 2, or at least wasn't implemented during its publication. However, the canon states that Stands have existed from ancient times, but are usually treated as "physic powers" in most parts of the world. Retcon aside, if we had to make up a reason Joseph's "Hermit Purple" wasn't visible to us, it'd probably be because we weren't viewing the characters from the eyes of Stand Users. There's a certain scene at the start of P3, where it shows the cops being unable to see anything going on during Jotaro and Avdol's fight. This is likely how we "see" during the first two parts.