The cube is built around balancing the weakest color pair. RW uses an equipment theme with many equipment cards doubling as creatures.

WU - Artifacts + Enchantments
UR - Prowess
GW - Tokens
GR - Trample/Trix
RB - Burn
BW - Lifegain
UB - Control
UG - Emerge
BG - Recursion

Overall there is a lot of cross-pollination. Equipment fit into the UW artifact theme, UR prowess theme, and are frequently token producers for GW. Black burn connects RB burn to BW lifegain. GR trix partially pollinated UR prowess. Lastly, UG Emerge and BG recur help eachother.

Since GR trix is an archetype, there's not a lot of low-costed hard removal at instant speed.

Last but not least, a few additional cards are made to make mono black viable.