  The emergence of syntax during childhood is a remarkable example of how
complex correlations unfold in nonlinear ways through development. In
particular, rapid transitions seem to occur as children reach the age of two,
which seems to separate a two-word, tree-like network of syntactic relations
among words from a scale-free graphs associated to the adult, complex grammar.
Here we explore the evolution of syntax networks through language acquisition
using the {\em chromatic number}, which captures the transition and provides a
natural link to standard theories on syntactic structures. The data analysis is
compared to a null model of network growth dynamics which is shown to display
nontrivial and sensible differences. In a more general level, we observe that
the chromatic classes define independent regions of the graph, and thus, can be
interpreted as the footprints of incompatibility relations, somewhat as opposed
to modularity considerations.
