Three Hearts was the trope in the 80s. Health bars came in later - 90s? The red colour, with it's obvious association with blood, has been around since video games had colour.
That's interesting. I actually feel like most health bars I encountered were either yellow or green or white. (EDIT: Back in the day, that is.)
I think the first time I saw a heatlh bar was for Street Fighter. I remember that in the original game, the health bars were fully yellow at full health. I can't remember if the version I played if the yellow part just got shorter to reveal a red background withn the bar or if the bar was transparent.
Some other games I played used either white or green for health bars. I can't remember if the orignal Fire Emblem used white or green. Chrono Trigger used green. There was this game where you got to switch between Ultraman, Gundam, Masked Rider, and some blue hair dude which I think used white health bars.
Anyway, you get the point.
The hp tokens have red hearts on them, so they also call back to those early video games.
Back then, I don't think I actually made the association between the red with blood, probably because the idea of blood freaked me out as a child.
I think the first time I made the connection was when I was played diablo 2, in the 90s. It was the first game I played where I saw my character actually bleed as their red health orb got depleted. Man, I really miss the feeling of diablo 2 for the first time.
EDIT: Do I have a point? I don't think I have a point. I'm just getting nostalgic. Wow, I feel like an old fart.