This is very annoying. I've completed one Support Colonization, one Defection, and one Exchange Mission. In all three cases, I was awarded a different rarity DOFF than what was promised on the completion screen:

(2 weeks ago) Bajoran exchange--promised very rare, given rare
(today) Support Zenas Expanse Colonization--promised rare, given common
(today) Investigate Defection--promised very rare, given rare.

WTF?

Mission rarity color =/= resulting doff color

The bajoran exchanges 2 weeks ago were bugged. It happened frequently that the wrong doff color was awarded on those assignments, but that's unrelated to the other two.

It's impossible to get a blue doff out of support. On critical success you get a white refugee and a purple doff, on normal success you get a white refugee.

You get a purple from instigate on critical success, you get a blue on normal success. Check your assignment log and make sure you read it correctly.

For support, you get a White (common) refugee on Success and for crit you get a Purple Doff and a White Refugee.

I'm guessing one of those things happened and you noticed the white.