All valid reports ("Report is VALID!") - both negative and positive tests - are in green color (Bootstrap 3 class .alert-success).
All invalid reports ("Report IS NOT VALID!") are in red color (Bootstrap 3 class .alert-danger). You can see it when open https://pcr.euprava.gov.rs without any subdir/script/query string.
Green/red have the same semantics in Serbia as in Germany.
This green for positive test mean just that QR code and associated test are valid, not that it’s good or bad. Positive test is bad in the first 14 days but after that it’s good - it gives you same rights as if you were vaccinated. But it’s the same test in both cases and green just tells us it’s a valid document.
Digital certificate is different kind of document. That’s the one you need to enter restaurans, bars, etc. after 20h - it will be red if you don’t fulfill conditions, e.g. vaccination or positive test within last 6 months (but not within 14 days), etc.
In programer’s lingo, test result is const, while digital certificate is a function of multiple conditions in context of current date/time.