Also worth noting is that the color only applies to sent messages. When you receive a message, it's just gray in either case. It makes a certain amount of sense to let the user know which transport their outbound message went on since it will affect your expectations.

I worry this a subjective matter, i.e. if the colours were reversed some people would make exactly the same complaint.

The actual argument really should focus on whether phone providers should use some interoperable standard more capable than SMS. If they can't come to consensus then the telecommunications regulators should involve themselves and force one.

In general, Apple has lowered contrast throughout the UI over the years. There's an accessibility setting for high contrast if you need it.

Yes, one could argue that the default should provide high contrast for everyone, but once this setting is enabled, it effectively becomes just that going forward for those that need it.