It's like mixing vodka and milk
A White Russian is a cocktail containing vodka and dairy (sometimes milk, but sometimes cream or Kahlua). This is a poor simile, since the listener has to interpret what is to be explained (how Prodigy and his opponent will interact) before being able to interpret the poetic comparison (how vodka and milk interact).
Some possible explanations:
Although this drink is chilled out and delicious, Prodigy does not seem to look on it favorably. He thinks vodka and milk don’t mix well, just as he and his opponent won’t.
His revenge will be sweet (and served cold) like a White Russian. In this case Prodigy does believe that vodka and milk mix, and compares his treatment of his opponent to the resulting mixture.
In the spirit of “an eye for an eye,” Prodigy believes that he and his opponent are perfectly matched (so that if his opponent brings a gun, so will he). In this case again, vodka and milk are thought to mix well.
A much less likely explanation is that he’s referring to how when you drink a mix of vodka and milk as in a White Russian, the next day you have terribly bad bowel movements, hence “It’s like mixing vodka and milk / I’m going out blasting”
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