I can only give general impressions and maybe some insight to issues in Greece as I understand them.
Yes, there have been some attacks/discrimination/issues with Greeks and minorities. Those, as I recall, were more prevalent when Greece was deep in economic troubles and there were riots in several cities. The stories concerning minorities really had more to do with minority immigrants and the perception that jobs were being stolen or services were being monopolized by those immigrants (think of the US and our own issues with immigrants, legal or illegal).
While things have settled down some, even then, as a tourist and obviously sticking out as (I can only assume American or Canadian) a visitor and in tourist destinations, I would not have great concerns. The Greeks are very aware that one of the few lucrative businesses still functioning is tourism and take great pains to assure that it remains so.
These are just my thoughts as a middle aged white guy, there may be some forums out there specifically for minority travelers that may offer actual experiences.