It does not appear to be possible to set the background colour of the BPMN swimlane.
However as an alternative, have you cosidered selecting the "Swimlanes..." option from the right click menu on a diagram or the "Configure Swimlanes..." option from the Diagram menu options. Both options lead to the same "Diagram Swimlanes" screen.
While creating diagram swimlaes it is possible to set the background colour and orientation of the swimlane.
Note however that unlike Swimlane elements from the toolbar, diagrams swimlanes are specific to the diagram where they were created and hence cannot be moved to other diagrams (i.e.) no reusability. That is the reason they do not appear in the project browser.
Another option is to create stereotypes using shapescripts and making the rectangle at the left end of the partition to appear filled with a colour. A messy and cumbersome approach but a workaround to meet your need that creates reusable partitions.
If you want to experiment with this approach here is the shapescript.
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shape main
{
noshadow = true;
//Create Rectangle for colour
setfillcolor(0,150,0);
rectangle(0,0,5,100); // (x1,y1,x2,y2)
//Create lines for rest of Partition
startpath(); // Start to trace out a path
moveto(5,100);
lineto(100,100);
moveto(5,0);
lineto(100,0);
endpath(); // End tracing out a path
fillandstrokepath(); // Fill the traced path with the fill color
}
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I suggest you consider issues like need, maintenance downstream and overhead before you adopt any option.
Please post here if you find another alternative.
Cheers
