So here is my theory: The end is a dream/vision. If you take the red ending, at the very end, with your EMS maxed out, you see what appears to be Shepard gasping for a breath, surrounded by rubble. No way does he survive the Citadel exploding like it does.
For the blue and green ending, watch him closely. You'll notice that his eyes and skin look like a husk. Like someone who is indoctrinated.
Couple that with Shepard's gun suddenly having unlimited ammo, random reapers being there to shoot and a mysterious wound in Shepard's left oblique (he got shot in the right shoulder) lead me to believe that everything after the run towards the light is a dream or halucination