The Communication Message Coloring page in the Options dialog enables you to configure the colors used in collaboration diagrams. When you enable this option, collaboration messages appear in different colors depending on the sequence group they belong to on a diagram; eg. 1.n are black, 2.n are red, 3.n are green.
Select the Use Communication Message Coloring checkbox to turn on colored messages.
Click on the down arrow in each color field, and click on the appropriate color for the message group. Set the color sequence as required; the pattern repeats after 8 sequence groups.