The three switches on the wall will turn on in a random order.  Shoot them in that same order to open the route to the yellow part of the room.

Here you will face a most terrifying monster: a ferocious, man-eating floor.

The monster will head straight towards where you’re standing.  You don’t want to still be there when it arrives.  (The monster always moves in a straight line until it reaches the edge of its domain.  It then turns and heads straight for you again.)

You can’t do much to harm the monster while it’s hiding in the floor.  Luckily, the marks on the floor that the monster sometimes leaves behind will make it show itself, if you can step on one quickly enough.  When the monster emerges, shoot it.  Do that twice and the monster will be defeated.

You can then walk from the monster’s area to the door on the right, lighting up the path as you go.