  We investigate a class of 2-edge coloured bipartite graphs known as
alternating signed bipartite graphs (ASBGs) that encode the information in
alternating sign matrices. The central question is when a given bipartite graph
admits an ASBG-colouring; a 2-edge colouring such that the resulting graph is
an ASBG. We introduce the concept of a difference-1 colouring, a relaxation of
the concept of an ASBG-colouring, and present a set of necessary and sufficient
conditions for when a graph admits a difference-1 colouring. The relationship
between distinct difference-1 colourings of a particular graph is
characterised, and some classes of graphs for which all difference-1 colourings
are ASBG-colourings are identified. One key step is Theorem 3.4.6, which
generalises Hall's Matching Theorem by describing a necessary and sufficient
condition for the existence of a subgraph $H$ of a bipartite graph in which
each vertex $v$ of $H$ has some prescribed degree $r(v)$.
