They're tolerant of other races, but not other cultures. ^^
If someone of Serenian descent is born and raised in Thanatos as a Thanati, they'd be cool with that person. They'd be considered a Thanati, even if they do have different colored skin and hair. If a naga is born and raised in Thanatos, it's all good by them!
However, if a Serenian came to visit from Serendipity, that's different. They don't like foreigners. xP It has nothing to do with their race.