Well, the reason you can make that question difficult is because so many other types of bigotry, preferences, an actions have been recently conflated into "racism." Anything and everything gets called "racism" these days.
Racism is bigotry that is the presumption of moral and intellectual characteristics based upon phenotype. That was invented in the 16th century along with modern biological taxonomy.
Ehhh...
In ancient days, bigotry more often rested upon cultural differences. Phenotype was given aesthetic consideration, but people did not presume moral and intellectual characteristics by phenotype.
Ehhh....
A Roman citizen would be confused to see two people who had been raised in the same city, spoke the same native language (even with the same accent), ate the same cuisine on their mother's knees, went to the same school, worshiped the same gods, fought in the same army...yet claimed to be the same cultures as people from greatly different countries in which they had nothing in common but phenotype.
Ok...
What you're calling "cultural differences" I'd probably call "magical explanations" for
any differences. People understood that others were different or in ways
inferior but the reasoning used to explain such things was rarely rational or grounded in a rational understanding of reality. Instead it's attributed to their gods, their rituals, the profound lack of understanding and magical beliefs regarding all things foreign.
So when we talk about European ascendancy, western civilization's eventual dominance over other cultures....I think it's important to remember these two points.
1. It's always been the way of all mankind to see itself in terms of in groups and out groups and tend to see and explain the out groups as something lesser, or inferior, abhorrent, or otherwise reviled.
2. At the time of European imperialism or colonialism...there had, arguably never been a wider gulf between the wealthy and advanced (or simply powerful) people of the world and....everyone else.
So it's hard to really understand how it must have looked to people on both sides of the equation. To a lot of foreign cultures which had never seen a European, let alone a horse, or oceanic galley, never seen white skin or blue eyes or red beards....it's not hard to imagine that one of two reactions was common. Fear or wonderment....probably a mixture of both. To the European explorers...it would have been hard to imagine it as much more than...disappointing. There were no magical cities of jade or gold....no great undiscovered empires to trade with (at least not as great as India or China), no half human half wolf people, no shapeshifters, no magicians, no fountains of youth or people with tail or fish scales. Instead, everywhere (apart from Africa...which would remain mysterious and dangerous for some time) a similarly brown colored people who seemed perpetually stuck in some primitive past that was 2000-3000 years behind the Europeans.
I say disappointed because while these may later have been described as "explorers" what they were really searching for was
value. Valuable trade routes and goods that would give them advantages over European competitors. There was very little value in the discovery of potatoes and corn initially. Civilizations like Incas and Aztecs may have provided initial caches of gold, those dried up quickly.
Europeans were increasingly methodical and rational though. They were losing their mysticism while at the same time increasing in rationality. That's the proper context for where modern ideas of race and racism come into play. The brightest minds of the day figured there must be a rational reason for why no one was advanced as they were, and everywhere brown people died in great numbers to diseases. It's that juncture of mythical Christian superiority made rational through the modern biological methods of the day.