Let's go through the list:

> 1) Every function has a color

Every function either throws an exception to indicate failure or doesn't. There's actually several different function colors available here, based on how failure is indicated: throwing exception, aborting the process, composite return value, error code return value, global errno-like variable, error code as a parameter, ....

> 2) The way you call a function depends on its color

See above.

> 3) You can only call a red function from within another red function

Some of the failure methods, like aborting on failure, cannot be converted to another mode at all (or only with very great difficulty). Others, like exceptions and errno-based routines, come with environmental constraints that could be contained by an error conversion routine in theory but may be precluded due to how the system as a whole works (e.g., a global variable errno doesn't play well with threads). Which isn't quite the same thing, but then again, "red function" here is async function, and the call-async-from-sync variant is the easier one to pull off (you spin the event loop), and has roughly the same issues as trying to box an exception routine: it only works if the system as a whole has mechanisms to make it work.

> 4) Red functions are more painful to call

Okay, you've got me here... the exception routines are the easier ones to call, syntactically than non-exception-based ones. Internally in the optimizer, however, exceptions are definitely the worst form (even errno somehow ends up working out better, and that's also deeply problematic).

> 5) Some core library functions are red

Oh yes, standard libraries love using a mix of all of these error-handling routines. Look up C++ <filesystem> for example.

Failing is a color, but throwing an exception isn't. An exception-throwing computation can easily embedded in a computation that doesn't throw - you can catch and return null, etc. But very rarely can a computation that may fail be part of a computation that may not fail.