BBC programmes: write to them with the specific programmes and as much detail as you can wrt time within the programmes where this happens. Include that it's a Foxsat-HDR and the South BBC One that you watch/record.
Specifics are important to aid fault-finding / tracing.
Access services are important to BBC and they wouldn't want this sort of error to occur, even if it is a specific coding issue only affecting one service via satellite and not via terrestrial (and maybe even one specific receiver).
BBC South is on a shared transponder (not BBC services only, and even includes $ky encrypted services) - but that won't account for you observing the same on other services (inc +1 versions).
As I don't use subs normally I can't readily confirm/deny the same on my version of BBC ONE for the same programme, if I had it recorded. Although your tests do seem to show that your Humax has an issue with the colours on DVB subs only, my brief experiment did not as both DVB and teletext had the same incorrect colour issue.
If interested the BBC publishes all sorts of useful information about subtitling for the Industry:
https://www.bbc.co.uk/accessibility/forproducts/guides/subtitles/
Other broadcasters may or may not be as interested (though some will be). So it may be worth writing to those, as well?
Note that if it is a Humax issue (e.g. non-compliance with a subtle part of the spec) it will probably never be corrected as Humax have dropped support for the product long ago.