The fact that echo green writes to stderr mainly just means that you can see the non-determinism happening, because if it wrote to stdout its output would be invisible.

The big part that's not deterministic is whether echo red succeeds or dies, along with which order the programs exit in. That would be nondeterministic even if you just ran "echo red | echo blue". But in that case you would always see "blue" so it would be hard to tell.

In powershell, it would be deterministic. It sounds like echo red would always succeed.