White has effects that give temporary small bonuses to one or more creatures (often +1/+1). A lot of those hit your entire army at once. This reflects white's ability to act as an entire army. White also uses enchantments to pump its creatures. An iconic example is Crusade
Black is also not a great fan of pumping. However, it does happen. When black pumps its guys it's usually by adding power and decreasing toughness. This reflects black's tendency to pay any price for power. The exception to this is shades, which allow you to pay mana to usually give the shade a corresponding power/toughness boost. They are a recurring theme in black.
Red usually has power-boosting. Red creatures tend to just charge blindly into battle an ignore their defences. This is mostly reflected in the Firebreathing-ability, which increases power according to the amount of mana spent.