The best, most fundamental way to do this would be to start off rather early on, and with a POD you may not expect. Namely: with the Spanish colonial empire. Spain produced the School of Salamanca, which was radical for its time: it advocated the fundamental equality and rights of all human beings, opposed the depradations of secular colonialist ventures, argued that slavery was immoral, reasoned that every person in the Spanish Empire who converted to Catholicism should have the same rights as any Spaniard, supported freedom of conscience, supported limiting the power of the Crown (insofar that it should be forced to respect the rights of all subjects), and embraced a very scientific approach to the Scholastic tradition.
There's a reason why the USA had laws against race-mixing well into the 20th century, whereas the former Spanish empire has a population that has mostly been mixed for a very long time. This does not by any means imply that the Spanish Empire wasn't racist, but that it already had a sound basis to overcome that. In fact, it was mostly Spain itself (and its colonialist rulers) that enforced the caste-like system (rule of thumb: "the whiter, the better"). The people born in the colonies cared far less. So if, for starters, you bolster the School of Salamanca and its beliefs within Spain itself, that goes a long way to improving things.
Regarding temporal power, Spain also saw the idea of the Aranda Plan raised. After the ARW, the Count of Aranda proposed turning the Spanish Empire into a federal entity, with broad powers of self-governance for the overseas parts. Based off the aforementioned POD of a stronger School of Salamanca, this could end up being carried out. This is bolstered by the fact that Aranda supported free trade within the Empire, treating all parts as equal-- and the School of Salamanca likewise advocated free trade over protectionist policies, as well as the equality of all (Catholic) peoples.
The end result could easily be a surviving and thriving Spanish Empire, which has decisively turned its back on racist ideas, and which has a leading philosophical attitude that is open to the free exchange of ideas. Its policies of racial equality, as well as equality of all parts of the Federal Empire, would no doubt benefit them immensely. This would then force other European powers to either imitate these intelligent policies, or fall behind. Thus, the ATL 19th century would see a consistent decline in racism, and a consistent move away from imperial domination and towards true federalism. Not least because wherever such things would be implemented, they would have positive effects, whereas powers the remained staunchly opposed would only be hurting themselves in the process. They would literally be outcompeted by the superiority of non-racism and federal co-operation over racism and imperialst exploitation.
The School of Salamanca, which extolled the virtues of such free competition (not only of merchants and nations, but also of ideas) from the very start, would no doubt be immensely pleased by this outcome.