Discolored

Go upstairs in the diner and use your viewer on the TV.

You'll need to get to the attic of the diner, which requires solving a some puzzles first. Then, use your viewer around the bike's handlebars and the bed.

Puzzle walkthrough:
Go to the left of the outside of the diner to find and pick up a bucket.
On the right of the outside of the diner, use the bucket in the well, use the well (putting water in the bucket), bring the bucket back up, grab the water-filled bucket and hook
Go upstairs in the diner to the plant with the green triangle. Use the water-filled bucket on the plant. Pick up the viewer lens that comes out as a result.
Use the viewer lens on the the cash register downstairs. Pick up the green prism.
Use the green prism on the green triangle upstairs. Grab the bar that appears above the plant as a result.
Combine the bar and the hook in your inventory. Use this new item to open the door to the attic above you.
Go up to the attic. Now, use the viewer in the appropriate spot (see picture above) and the achievement is yours.

On the left side of the diner (from when you enter it), use your viewer in the corner by the chairs.