The Reopen EU website is a far better resource than this forum about Interrail/Eurail travel. If you want to be sure, go to the Reopen EU website and click through to the national websites because they will have the latest information.

As to the entry requirements, there is no general answer. Every country has its own entry requirements and they usually depend on where you've been in the past 10-14 days, the vaccinations you have and sometimes where you live or some other parameters. Some countries still have their own colour system. A colour red does not have a meaning without knowing who calls it red. Requirements also change frequently and with little notice so always check again shortly before travelling.

For US, EU-people, red means that many insurance is not valid and that ´non-essential´ trips are discouraged. But for EU-people there are no red zones IN the EU. Again: rules can change by the day if not hour and as rain explains, it depends on many more factors.

That also depends on the country's colour system. You can't make a general statement like that.