How to change line color automatically in rxgraph?

edit

Because when rxgraph show many lines on the screen,sometimes it's difficult to each line of goal.

If it is possible to change color automatically,it will be easy to understand.

The normal black line:

I use grep to find color,but what I found is not black:

 sam@/opt/ros/diamondback/stacks/rx/rxgraph/src/rxgraph$ grep color *
 dotcode.py:            return '  %s [color="red", shape="doublecircle",      label="%s", URL="node:%s"];'%(
 dotcode.py:            return '  %s [color="orange", shape="doublecircle",      label="%s", URL="node:%s"];'%(
 二進位格式檔案 dotcode.pyc 符合

 sam@/opt/ros/diamondback/stacks/rx/rxgraph/src/rxgraph$

Where is the setting of line of color?

Thank you~