Well it was shown that Picard could still hear the collective in First Contact so he did have something left in him. But he was a different drone all together.As others have said, the Doc simply stimulated their hair follicles as he did previously with Seven of Nine. As for getting the colors and styles right, I imagine he simply used whatever pertinent data from the last time they'd used the transporters.
What puzzles me more is that it was said that the Doc had removed most, but not all of their implants. What implants were these that they had to keep and where were they? Obviously somewhere under their clothing where they'd not show. And I imagine they'd have to retain a certain amount of nanoprobes in their systems permanently, though lying dormant - Picard, too. I also wonder if they needed to use the regeneration alcoves, even for a short time.
I always figured that he couldn't remove all of Seven's technology because she was assimilated as a child so her body grew with those implants and relied on them. The others were assimilated as fully developed adults so I assumed it was easier to remove everything.
though I do think it would have been interesting to see some lingering effects, some residual implants or an episode where the three of them shared a mini collective.