The colour of a glider is a property of the
glider that remains constant while the glider is moving along a
straight path, but that can be changed when the glider bounces off a
reflector. It is an important consideration when building
something using reflectors.
The colour of a glider can be defined as follows. First choose
some cell to be the origin. This cell is then considered to be
white, and all other cells to be black or white in a checkerboard
pattern. (So the cell with coordinates (m,n) is white if m+n is
even, and black otherwise.) Then the colour of a glider is the
colour of its leading cell when it is in a phase that can be rotated
to look like this:
A reflector that does not change the colour of gliders obviously
cannot be used to move a glider onto a path of different colour than
it started on. But a 90-degree reflector that does change the colour
of gliders is similarly limited, as the colour of the resulting
glider will depend only on the direction of the glider, no matter how
many reflectors are used. For maximum flexibility, therefore, both
types of reflector are required.
Small periodic colour-changing glider reflectors (bouncers) are
known, and also small periodic colour-preserving glider reflectors
(bumpers). Among stable patterns, only a small colour-preserving
reflector (Snark) is known. The smallest known 90-degree
colour-changing reflector is given at the end of the reflector
entry.