It is important that the mass of each fluid should remain constant in such a simulation. Thus we must add the conservation of colour equations
to the conservation of mass and momentum equations (3.4) and
(3.5). Here 
and 
are the occupation
numbers for red and
blue particles on link 
. We can define
to be the colour density given by
Equation (3.42) represents the only restriction on the colour of the
particles. There is no constraint imposed, after a collision,
as to which of the particles
at any site should be of which colour.
The available colour is re-assigned
arbitrarily amongst the particles at each site after a collision. Thus
if three particles, one red and two blue, arrive at a site and collide then
after the collision one particle is randomly labelled red and the other
two blue.
