I don't think non-whites were excluded from purchasing first class tickets, but their shipboard experience would vary depending on their social status and the personal prejudices of the passengers (for example, Booker T. Washington, Ida B. Wells, and Mary Church Terrell traveled to Europe aboard ocean liners and since Washington and Terrell were wealthy and renowned, they probably sailed first class).
I recall a scene in Elizabeth Drexel Lehr's memoirs where an African ambassador was eating at a top hotel restaurant in London (could have been the Savoy), and an American guest was horrified that a black man was eating in plain sight. The waiter could not expel him, despite her protests, because he was important. Ultimately, there was a screen placed between them so the woman wouldn't be offended.
If the crewmen loading the boats has shown any favouritism for passengers with an ethnic background most like their own, then in 3rd Class we'd expect the large contingent of British passengers to have had the highest % rate of survival. In fact they had the lowest, while among the highest rates of survival we find for instance the Lebanese and Chinese.