Why is white 2 useful at the start?

It returns almost immediately, creating the hole used by cyan 21 without delaying the lime and dark openers.

The upper purple horns and their pale-cyan border remain after the body, legs and wings disappear.

Colony Flow! level guide

Colony Flow! Level 398 Walkthrough

medium 6 colors

Pair the dragon's many 35-count body stacks with short eye and outline cards, clearing the cyan field before the isolated purple horns.

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Launch lime 17, white 2 and dark 20; reuse the white hole for cyan 21.
Route cyan 17/15 and one pale-cyan 35, then use purple 38 only after dark 20 returns.
Work lime 15 and cyan 10/14/34 through the field, keeping two repeated 35 stacks queued.
Alternate dark 35, purple 35 and pale-cyan 35 as the field recedes instead of running three long routes together.
Clear the legs with purple 26/18, dark 9/6 and cyan 34; wait for purple before another dark 35.
Use lime 10/17/1 and white 1 for the eyes while purple 3/25/22 removes the inner body.
After the body disappears, route dark 16/29/21, purple 25/15/9 and cyan 34/19 through the head and wing remnants.
Finish the horns with dark 9, purple 2/2 and pale-cyan 7/7, then clear the remaining field border.

Colors in this level:

Purple, Pale cyan, Cyan, Lime, White, Dark purple

Starting three visible 35-count cards together delays the next short lime or white detail.
Stopping when the body clears leaves two purple horn islands inside the cyan field.

Board notes 5 details

Layout: A purple baby dragon with lime eyes and cyan wings sits on a pale-cyan field. Five holes face lime 17, white 2 and dark 20 above pale-cyan 35, cyan 17/15 and repeated dark, purple and pale-cyan 35 stacks.
Goal: Strip the pale-cyan field, then clear the cyan wings, purple body and horns, lime eyes, white highlights and dark outline.
Opening: Start lime 17, white 2 and dark 20, reuse the rapidly returning white hole for cyan 21, and delay another 35-count stack until dark 20 returns.
Danger Zone: Three simultaneous 35-count body routes can block the short lime eye or white highlight card required by the next exposed section.
Mechanics: A deep repetitive 35-stack queue replaces locks and gates; the field, legs, wings, body and horn islands clear in distinct visual phases.

Why is white 2 useful at the start?
It returns almost immediately, creating the hole used by cyan 21 without delaying the lime and dark openers.
What clears last?
The upper purple horns and their pale-cyan border remain after the body, legs and wings disappear.