As the Reanimation strategy isn't something new to magic, many decks in all formats have been built to use it - including in pauper.
However, In the pauper format we only possess two effective reanimation spells: Unearth and Exhume.
This means every reanimator deck in pauper should run black (therefore, when building and brewing reanimator in pauper, One should always include black as the main color).
But why Red? To answer that, well need to look at what reanimator does:
Reanimation strategies in magic use the Two-step combo line: get your fattie/target in the graveyard, then reanimate it. Since in our deck Black does the second part well but doesn't quite do it for the first part (there ARE exceptions), we need a support color that's good at discarding. Obviously, Blue and Red.
While Blue has access to cards like Careful Study, Tolarian Winds and Thought Courier, they are mostly considered slower (being the color of control in magic) and less efficient than the discard outlets Red has. Of course, you can build a great Dimir reanimator deck, but since the discard outlets red has to offer are better, Rakdos is considered the more competitive version.