AMA0 should be correct. Make sure serial is enabled on pi and not used for console 0, but should not in our image.
Also make sure wiring is correct RX to TX and vice versa!
red/yellow means port is seen and it tries to connect. Means nothing about wiring/linux config correct. If it is correct the only errors left might be wrong baud rate, wrong firmware or printer not listening on the serial as expected. Also connect a gnd line from pi to printer so both use same potential. I think that is important bit electronics is not my field.