Dark-colored clothes absorb light thus retaining more heat as opposed to light-colored clothes which reflects light.

 

So let me ask, what if person A is wearing a grey sweater and a white undershirt. And person B is wearing a grey sweater and a black undershirt. Would B be more warm than A even though the undershirt is covered by the sweater and is not hit directly by the sunlight?

i would go for no, it is the grey sweater that is doing the absorbing and reflecting. ultimately it is the grey sweater that controls the energy input. after that the heat is no longer transmitted by electromagnetic radiation in any considerable amount.

if no light gets to the other side of the outer gaurment it should make no difference, if some light does get through it may make a small difference...