We are on Earth, in Vancouver.
We see the Child, playing with a ship.
The area is grey, similar towers everywhere, with fire raining down.
When knocked back onto the floor, we hit a bench. In the background (this is confirmed by audio isolation) plays the dream music.
We see the Child killed by the Reaper Destroyer.
We leave on the Normandy.
We go to the Normandy.
We have a dream, where Shepard has armor and weapons.
He follows the Child, sees the bench, and is shocked by the Child burning.
He wakes up in his bed.
~~~
We are in Tuchanka.
The area is yellow, brown for the most part. Ruins.
We see a Reaper Destroyer. It is poisoning the atmosphere with a green toxin, appearing like a wispy beam.
We kill the Reaper Destroyer by baiting a giant Thresher Maw.
The sending out of the Cure or fake Cure turns the green into yellow.
We go to the Normandy.
We have a dream, where Shepard has only his armor.
He follows the Child in the darker forest, passes the bench, and is shocked by the Child burning.
He wakes up in a chair, with a datapad possibly still in his hand.
~~~
We are on Rannoch.
The area has a red hue, but also brown again. Ruins.
We see a Reaper Destroyer. It is controlling the Geth through upgrades. (The consensus illustrates records with a green circuitry, but the virus is more red)
The Reaper is inside some kind of hole. We see similar locations at the Prothian Mars ruins, and the multiplayer Firebase Rio.
We kill the Reaper Destroyer by assisting the Quarian Fleet in its targeting.
Legion/Geth VI attempts to send his own upgrades to the freed Geth through an interface that is a blue sphere with a more organic red core.
We go to the Normandy, possibly with our Love Interest.
We have a dream, where Shepard has only his armor.
He follows the Child in the darker forest for the shortest dream.
He sees another Shepard, this time without armor and weapons and only in casual wear.
This other Shepard embraces the Child, looks happy yet the image is also creepy. They burn in fire... but we don't get to see our Shepard's reaction.
He wakes up in bed, to the possible reassurance of his Normandy Love Interest.
~~~
We are on Earth, in London.
The area is very, very dark, but the beam in the distance provides a light blue that illuminates all around it. This time, the ruins are everywhere, and recent.
We see a Hades Cannon. We get to destroy it with a Cain.
We see a Reaper Destroyer. It is blocking our path to the beam, which we need to reach in order to enter the Citadel and activate the Crucible.
To defeat the Reaper Destroyer, we have to lead it towards us, and away from the.. disrupting signal of the beam. This way, we can actually still kill it with weaponry. We do so, with the covering fire of allied forces.
We enter the beam, or as the objective markers and EDI call it, the 'conduit'. (skipping TIM stuff here)
We enter a dreamlike area, while we hear from the child (and soundtrack) 'wake up'.
Shepard's armor is still there, but burned. Shepard still has a weapon, but it is a basic pistol.
He speaks with the Child, who is really the Catalyst (or to the Leviathans, an Intelligence). He doesn't embrace the child, but he also cannot directly shoot him and directly proceed.
~~~
We can pick red Destroy, shoot our gun towards an outstanding tube, face the explosion with our armor intact.
Or we can pick blue Control, keep our armor until it also disintegrates, lose our gun as we grasp for handles..
Or we can pick green(ish, we at least turn green) Synthesis, disintegrate our armor quickly, drop our gun as we run forward.
These options each may or may not be available, but the interfaces to use them will always exist. Only the paths may not be open. (You could say they are 'greyed out', in a way)
You can consider this only symbolism, and that's totally fine. At least in itself, ME3 works fine with that easy and possibly entirely true interpretation.
But man, you have to admit that when collected, this is pretty damn heavy handed symbolism, at least?