The use of colour in graphical representations of data has long been recognised as a useful tool in the graphical display of information [11,20]. The advantage of colour information over other distinguishing features is that it is so abstract. Although colours such as red and green are traditionally associated with concepts such as stop and go, in general colours can be assigned to concepts completely arbitrarily, provided that consistency is maintained. In keeping with the principle stated above that the environment should allow the maximum freedom to programmers to convey information however they feel is best, the colour of objects should also be arbitrarily assignable by the user. Thus, the user has an additional facility to convey the relationships between the objects.