The purpose of this study is to show how coloured Petri nets can be used to select the crossing rules guaranteeing that two ships avoid collisions at sea in accordance with the international regulations on this matter. This paper is exclusively focused on the solution to this small sub-problem within the overall “Collision Avoidance System” that the authors of the present study are currently developing. For easier understanding, the overall system is also briefly presented. How the Petri net in the CPN Tools software is fed with “Real Time Real World” data is presented. These data are generated outside the Petri net from the function block to predict a possible collision and from the current meteorological data. We also demonstrate how the rule selections made by the Petri net are transferred from the CPN Tools software into the “Real Time Real World”. This transferred information is used outside the Petri net in the function block to calculate avoidance routes. The definition of the colour sets that are used and the individual operations applied to these colour sets in the coloured Petri net are presented.