Hi @Giuseppe_Zarbo and welcome in our forum!
This is not a trivial thing to do, because you first need the graphical SVG representation of the connection:
var outgoingGfx = elementRegistry.getGraphics(outgoing);
Then you can set attributes of the connection path. To change the color you can try this:
outgoingGfx.select('path').attr({stroke: 'blue'});
A more general approach (i.e. if you want one specific color for all sequence flows) would be to change the color in the BpmnRenderer. One way to do this is to add the stroke
attribute for the desired elements (in this case all sequence flows) here: