Which `3` can the player tap?

The round left source is tappable; the small `3` on the upper road falls when groups complete.

What opens the two lower doors?

Yellow completion opens the left yellow-key door, and the lower green completion opens the right green-key door.

Its visible stages release green, blue, beige, and a final lavender pair as the counter empties.

Car Sort level guide

expert 10 colors

Car Sort Level 503 separates an upper completion counter from a tappable round `3→2→1` queue while yellow and green keys open two lower stacks.

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Release the middle cyan pair, then route the newly exposed red and yellow cars into their visible receivers.
Complete the tall yellow group and stabilize the two red-toned middle groups, lowering the upper counter from `3` to `2`.
Open the yellow-key lower-left door and expose its brown and deeper blue layers without filling the road.
Tap the round `3` source for green; after the release it shows `2` with blue ready next.
Complete the lower-center green column to trigger the green-key door and reveal cyan and beige layers on the right.
Tap the round source at `2` for blue and let the lower-left receiver take that traffic.
Use the `1` stage for beige, route it to the lower-right receiver, and clear the queue's final lavender pair.
Finish middle red, burgundy, and yellow, then complete lower blue, green, and beige.
Resolve the upper hidden columns as pink, brown, lavender, and cyan and drain the last road traffic.

Colors in this level:

Cyan, Red, Yellow, Burgundy, Green, Brown, Blue, Beige, Lavender, Pink

Open the yellow-key door before the first round-queue tap.
Use the queue's green pair to complete the lower center and unlock the right door.
Wait for blue to enter the lower-left receiver before exposing beige.

Board notes 5 details

Layout: A two-tier `0/16` road links four hidden top columns, mixed middle stacks, a round left `3` source, and yellow- and green-key doors below.
Goal: Complete the middle red, burgundy, and yellow receivers, open both keyed doors, empty the round queue, and finish the top and lower color columns.
Opening: Dispatch cyan from the middle, then route the revealed red and yellow cars until the tall yellow receiver completes and the yellow-key door opens.
Danger Zone: The road repeatedly reaches `16/16`; a numbered-source tap at full occupancy adds a new color before its lower receiver can accept it.
Mechanics: A completion-driven upper `3→2→1` counter, a distinct tappable round `3→2→1` dispatch queue, two colored keys, and deep `?` layers stage the board.

Which `3` can the player tap?
The round left source is tappable; the small `3` on the upper road falls when groups complete.
What opens the two lower doors?
Yellow completion opens the left yellow-key door, and the lower green completion opens the right green-key door.
What colors leave the numbered queue?
Its visible stages release green, blue, beige, and a final lavender pair as the counter empties.