Which long colour should start first on Level 210?

Clear the dark 34 and brown 20 front traffic first, then keep every 45/50 route isolated while the orange 23 centre remains available.

Why does the orange diagonal get blocked?

Two fresh 45/50 routes consume the buffers that short green/yellow cards need to open the centre.

Colony Flow! level guide

hard 7 colors

Open the dark 34 and front brown/pink/yellow cards, then stage the 45/50 lanes one at a time around the orange diagonal.

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Clear the dark 34 and brown 20 front traffic before loading multiple long routes.
Use pink 45/50 and yellow 40/45 as single staged lanes, not a simultaneous block.
Keep green 9/40 and orange 23 available to open the central diagonal.
Rotate red 45 and the remaining pink/yellow 45/50 cards one at a time.
Finish orange diagonal, dark outline and small green/pink tails after the long cards are short.

Colors in this level:

Dark, Pink, Red, Yellow, Brown, Green, Orange

Keep two slots open at every 45/50 handoff.
Use the short brown 20 and dark 34 traffic before loading more than one 45/50 route.
Keep the orange 23 centre accessible while the 45/50 lanes drain.

Board notes 5 details

Layout: Red-and-pink spotted butterfly-like object with two orange diagonals, green accents and dark outline. The opening shows dark 34 above brown 20, pink 45, pink 50 and yellow 40; below are yellow 36/45, red 45, pink 59, green 9, orange 23 and green 40.
Goal: Clear the spotted body, orange diagonal, green accents, yellow highlights and dark outline until the board is blank.
Opening: The first active field is dark 34 with brown 20, pink 45, pink 50 and yellow 40; the 45/50 routes must be staged around the orange 23 centre.
Danger Zone: Pink 45/50, yellow 45 and red 45 fill all five buffers if fresh long routes are started together.
Mechanics: High-count dense board where the orange diagonal opens only through serial 45/50 handoffs and short green/yellow support.

Which long colour should start first on Level 210?
Clear the dark 34 and brown 20 front traffic first, then keep every 45/50 route isolated while the orange 23 centre remains available.
Why does the orange diagonal get blocked?
Two fresh 45/50 routes consume the buffers that short green/yellow cards need to open the centre.