When using a lattice gas model it is possible to distinguish between the
individual `particles' by labelling them.
Little can be gained from watching just one tagged
particle since it will travel at constant speed along the lattice
links
and may well be travelling in a different direction to the mean flow.
In lattice gas models it is necessary to consider the average behaviour
of a number of particles; tagging particles can only give useful
information about a flow if a significant percentage of the particles
are tagged. The simplest example of tagged flow is when the particles
are considered to be of two types or colours. In this case the particles
are initially labelled either `red' or `blue', say, and the
evolution of both fluids can be seen, along with any interactions
between the fluids. The idea of using two different particles in a
lattice gas model was first introduced by Clavin et al.
[25]. The model has also been extended to allow three different
particle types [26, 27].