Colourless, also known as Null (and formerly known as white cards), is a sort of neutral energy used in the vast majority of every card in the game.
Colourless is sometimes referred to as grey or neutral.
Colourless cards also don't have a pack of their own.
Colourless cards are meant to be played in every deck, but not to be played as a monocolor deck.
Black cards, which are cards with a black background or border, are considered to be white cards. However, black cards cannot be discarded for icons or generate icons.
Cards can be easily used in almost any deck.
Colourless cards benefit almost every deck by generating colourless icons.
Penalty to cost.
Limited to only a few effects and cards compared to other colours.
Very hard to be played together
Cannot be sent to the baseplate by normal means.
Take note that colourless cards aren't supposed to be played as a monocolor deck. Hence, the lack of variations for monowhite.