ITs not documented well...how to find..
When in Mech bay Select the Shadowhawk to retrofit.
Notice at the bottom left of the details pane the Paint Scheme, look directly at the top of the details pane, you will see what look like a another selector that has i belive the word 'standard', click it and selet umbra... there ya go!
It took me ahalf hour to notice the second selector as i though the lable box was simply saying standard untill i noticed the left right arrow selectors.