How does GHOSTDAG classify blocks in the blockDAG?

GHOSTDAG assigns every block one of two labels — 'blue' or 'red' — based on a measurement called Anticone size. As a DAG (directed acyclic graph) consensus mechanism, GHOSTDAG needs a formal rule to organize the many parallel blocks that can exist simultaneously in the network; the Anticone size constraint is that rule, and the blue/red result is how each block's relationship to the agreed consensus is recorded. For a beginner, this classification system is the key reason GHOSTDAG can maintain consistent network-wide agreement across a blockDAG rather than a single chain.