I've always found the blink while charging, solid when finished pretty easy to remember. Seems somewhat intuitive and pretty standardized over many different types of products, not just chargers.

Equally easy is red (or amber) for charging, green for finished. Single 2-colour or tricolor LED.

But that's state display. 1 LED is also fine to display functioning versus error state. But using the same LED for two different functions or errors is completely different.

With my current and last printers, I've gotten a report back on the computer indicating what the issue is - load paper, jam, etc. And an LCD on the printer itself explains the problem as well. Thankfully my printer wasn't designed by Google.