I was looking through some random pages at fallout.wikia.com and I had came across Vault 19, a vault in New Vegas. The vault was split into 2 groups, red and blue.
This was an experiment to test racism in humans. The vault had 2 Overseer's, one for each colour and had seperate offices.

Down to the point, I have heard that ghouls, when killed, drop red or blue keycards. The residents of Vault 19 needed a keycard the same colour corresponding with the area they were designated to. After a sulfur leak, the residents panicked and abandoned the vault.

My theory I am putting forward is that some of the humans that escaped from Vault 19 are actually some of the Ghouls in FO3.

I have picked up Red Pass Cards off of ferals, not yet the blue ones though. That's an interesting theory though...I suppose they could end up ghoulified walking across the nuclear wasteland from NV to DC.

Those red and blue pass cards are actually pre-war metro season passes like the pass cards used in modern subways

That doesn't make since, what would ghouls from the Capital Wasteland be doing in the Mojave Wasteland? What would they be doing in a vault?

I forgot about that, they were called red and blue line. Are there white pass cards that I haven't picked up?

Having only played FO3 and soon to get the GOTY edition, with the add-ons. Vegas really has NO bearing on the circumstances in DC or vice-versa other than factional infighting perhaps? We're talking half a nation away, correct? Anyways, once I'm done with FO3 +, I plan to get the Vegas stuff, but honestly, I have a feeling, it will be at least another month of just walking around ( having completed everything), before I'm willing to give up my "explorations".

And ghouls, kill if they show red, befriend them if they show green....that's my rule.

What? The colour of their skin/clothing or something else?

he means colour of the HUD when h"highlighting" enemies the reticule turns red when there non hostile by default its green (but for me its amber )

Okay? That rule is a bit... obvious.

Exactly.

Well, sometimes obvious means killing everything...and then again, sometimes not.

I believe he was saying people from Vault 19 would go to the Capital Wasteland, not ghouls from D.C going to the Mojave.

Well, if you think about it, the Citadel is mentioned in conversation somewhere in New Vegas implying that there are ways to travel cross-country and some people do. Considering the hostilities toward ghouls it might not be so outlandish for a mini-exodus to occur from Vault 19, however the metro cards explanation is almost certainly the right one.

When I first got a couple of them I thought it was an Easter Egg about the old Doom game. You would have to find Red and Blue passkeys(and yellow ones as well) to unlock certain doors in that game.

Of course, having read this thread I think the OP's reasoning makes more sense.