In 1.16 friendly mobs have green bars, and everything else have red ones.
However starting from 1.17 the bars change to blue and purple(or is it purple and blue). When I want to change colors manually, none of the formats seem to match the rendered colours.

I have seen this reported a few times now but haven't had any luck reproducing it on my computer. By any chance are you using a Mac? There is a known issue that should be fixed in a few days, however it might only affect Macs.

No, I am using Windows. Both of the computers I have (both Windows) have the same problem, and only in 1.17.x.
BTW a screenshot with only Torohealth & fabric API:

OK, it might still be related. I will post back here when the version goes up. Hopefully it will fix it. Thanks for letting me know.

Please try version torohealth-1.17.1-fabric-12.jar, I am curious if it will fix this issue.

OMG appology for not seeing the message, the mail was somehow magically categorised as junk mail...
Yes, the problem is fixed, perfectly. Thanks a lot! 😀

BTW may I ask if there is any support of customising texture of e.g. UI through resourcepack?

That is something I would like to add, but I don't think it is possible on the current build. I started work on it here a long time ago: #24, however I think I could get further with it now. Feel free to create a new issue for this so that I will see it when I come back to release a new version.