Do I need to collect the yellow key separately?

No separate key pickup is shown as the objective; the key identifies the thief, and touching the thief completes the catch.

Why is the central box important?

Its top lets Mr Potato cross over the obstacle and drop onto whichever side the thief currently occupies.

Mr Potato level guide

hard 1 colors

Use the central box to intercept the yellow-key thief from above and catch it to complete the level.

Layout: Mr Potato starts left of a broad rectangular box. A furry thief carrying a yellow key moves around the box, with arched doors visible at the far left and far right.
Goal: Catch the furry thief by making contact; the VICTORY state follows the catch, so the background doors are not the immediate target.
Opening: Move to the central box and jump onto its top surface to gain a route over the thief's floor path.
Danger Zone: The box blocks a direct floor chase, and the thief can switch sides while the potato is moving; do not jump blindly without checking the key's side.
Mechanics: The central box is an interception platform. The yellow key carried by the thief is a moving location marker that shows which side to attack.

Use the top of the central box to switch sides instead of trying to run through it.
Track the yellow key when the thief is partly hidden behind the box.
Think in chain clears. The best move is the one that sets up the next two moves, not just the quickest current match.

Move right from the starting side and jump onto the top of the central rectangular box.
Watch the thief and its yellow key to identify the current left or right ground lane.
Jump down from the box toward the thief's current side.
If the thief moves around the box, climb back up and repeat the interception from the opposite side.
Touch the yellow-key thief; the catch triggers the VICTORY screen.

Colors in this level:

Yellow

Running at floor level and stopping against the central box gives the thief time to escape around it.
Jumping to a side without checking the key can land behind the thief instead of on its lane.

Do I need to collect the yellow key separately?
No separate key pickup is shown as the objective; the key identifies the thief, and touching the thief completes the catch.
Why is the central box important?
Its top lets Mr Potato cross over the obstacle and drop onto whichever side the thief currently occupies.