If you assume that people of color are at least moderately competent judges of their own interests, then the Republicans not attracting people of color should at least be weak evidence that Republicans show prejudice.

It is also weak evidence that the Democratic party is offering non-white voters benefits beyond those they offer to white voters. A model of a race preference Democratic party and a race blind Republican party works just as well as long as you assume Democrats are slightly anti-white.

It's weak evidence because there are other possible explanations-- perhaps people of color don't like feeling so strongly outnumbered so there's a stable situation which isn't a result of white racism.

See Thomas Schelling's Models of Segregation

In 1969 and 1971, Schelling published widely cited articles dealing with racial dynamics and what he termed "a general theory of tipping".[10] In these papers he showed that a preference that one's neighbors be of the same color, or even a preference for a mixture "up to some limit", could lead to total segregation, thus arguing that motives, malicious or not, were indistinguishable as to explaining the phenomenon of complete local separation of distinct groups. He used coins on graph paper to demonstrate his theory by placing pennies and nickels in different patterns on the "board" and then moving them one by one if they were in an "unhappy" situation.

Schelling's dynamics has been cited as a way of explaining variations that are found in what are regarded as meaningful differences – gender, age, race, ethnicity, language, sexual preference, and religion. Once a cycle of such change has begun, it may have a self-sustaining momentum. His 1978 book Micromotives and Macrobehaviors expanded on, and generalized, these themes[11] and is standardly cited in the literature of agent-based computational economics.