I've completed 3 colonial chain missions and have yet to get a single duty officer. Don't know if it requires a critical success or not to get them. It does eat up a lot of Doffs and commodities to do them so it would be nice to at least get something at the end of the chain even if it's a white Doff.... Anyone know what's going on with these chains?
The final step does indeed require a critical success to reward the purple doff but it doesn't require any commodities or other expendables. And since it's repeatable you can get as many purple doffs as you like from it as long as you get a critical success on that mission. Personally I repeatedly do the one in Hromi cluster since that doff is perfect for the Cardassian Officer Exchange mission.