In D&D terms those colours had a lot of underpriced spells and creatures. Imagine a 4ht level spell in D&D that combined lightning bolt and a level 4 cure spells (R+ W Boros colours), or low level summoning spells that conjured more efficient creatures (or the Summon Animal Spell being level 2 instead of 3). A Zoo deck was notorious for losing life via pain and dual lands so by turn 3 it was not to unusual to be 20-30% down on their life totals that were self inflicted. You get powerful spells but vs a R/G aggro deck you can be in a lot of trouble as they deal lots of damage like you do but have a smoother mana base.
The drawbacks of those spells are they are mult coloured so they can be harder to cast although in multicolour blocks that is mostly mitigated although you can still get colour screwed on occasion. The MtG colours are Red, white green, black, and blue (shortened to U).
Mixing R/W is like a multiclassed life cleric+ fire sorcerer or Evoker with unique spells. Green and black (Golgari) would be like a MC Druid+Necromancer, W/G is like a life cleric/Druid. A more balanced why of doing it in MtG would perhaps be guild feats that did things like add an extra dice of damage to heal and fire spells for boros (R/W) for example.
https://www.mtggoldfish.com/deck/589506#paper
The Zoo deck was full of animals (Wolves, hounds, apes etc) hence the name. This is an Izzet (R/U) combo deck AKA Dragonstorm.
http://tappedout.net/mtg-decks/makihito-mihara-2006-worlds-dragonstorm/
In 5E terms its like a ritual that summons 4 adult red dragons and you win the game straight away. When Ravnica was legal all the decks were multicoloured.