The colour-field surface tension model was devised by Rothman et
al. [28]. Let
, the colour density of link 
at time t and site 
on a grid, be given by
the difference between the number of red and blue particles on the link.
Due to the exclusion principle 
can take the values -1, 0 and 1.
The colour density at a site, defined by equation (3.43), is given
by
The local colour flux 
is given by
and is the difference between the red momentum and the blue momentum at
site at time t where the site is in state 
, the sum of the red and blue states.
The local colour gradient 
is given by
which is the microscopic gradient
of the colour density 
.
The work 
performed by the flux against the gradient is
The out-state
of any FHP interaction 
is then chosen such that
where 
are all the possible
collision outcomes.