Mine does that too. Since we got out fish from the same source and same place at the same time i can say whatever your fish is doing weird mine is doing the same too. hehe. Now on the real side, i see my Rhom changes eye coloration too and i have noticed they turn silver when they are a tad bit scared. Mine turn silver almost everytime when i walk into the house and start staring at him. His eyes also turn silver when i come close or start messing with the tank when he is not expecting it. But after he knows im looking at him or whatever, his eyes turn back to red.
My light bulb is broken so he been in natural light for the past 3 days. It seems that with natural light his eyes stay red more often than with a light bulb on. Or! it can be that he is use to my presnce so he isnt scared anymore.
He is probably scared when he first sees you because he'd be startled so his eyes turn silver. Then when you've been around for a while he gets used to you and his eyes turn red. When I die maybe I'll get reincarnated as a piranha. Then I'll tell all you guys what everything means......somehow.
I came home at night and turned on the lights in my room and noticed my rhom's eyes were already silver so I don't think it's because he's scared. When I woke up in the morning, his eyes were red again.
I bought a fish from the same batch as your's, I've been checking the eyes closely a few times each day, they've been red each time I've looked...sorry if this doesn't help, I'll keep you posted if I notice any changes.