Function render_failure_plain 

pub fn render_failure_plain(
    spec: &Spec<'_>,
    argv: &[&OsStr],
    error: &Error<'_, '_>,
) -> String

A parse failure, never coloured.

render_failure asks the environment whether to colour, which is right for a process and wrong for anything that keeps the string: a test that asserts on a message, or a snapshot of one, would pass or fail by whether stderr happened to be a terminal. The renderer is the same; only the answer to that question is fixed.