Once a
gel is cut, it no longer has it's manufacturers identification label on it for colour. Therefore they should be labelled to ensure that all of the same colour gel's remain together. Using a Chinagraph (wax based) pencil, the colour number and manufacturers name(if more than one is used) can be written on each gel. If the numbers are big enough and central, then they can be seen and checked from the ground.
from
http://www.the-lighting-crew.fsnet.co.uk/intro.htm