factors←{⎕ML ⎕IO←1          ⍝ Prime factors of ⍵.
    ⍵{                      ⍝ note: ⎕wa>(⍵*÷2)×2*4.
        ⍵,(⍺÷×/⍵)~1         ⍝ append factor > sqrt(⍵).
    }∊⍵{                    ⍝ concatenated,
        (0=(⍵*⍳⌊⍵⍟⍺)|⍺)/⍵   ⍝ powers of each prime factor.
    }¨⍬{                    ⍝ remove multiples:
        nxt←⊃⍵              ⍝ next prime, and
        msk←0≠nxt|⍵         ⍝ ... mask of non-multiples.
        ∧/1↓msk:⍺,⍵         ⍝ all non multiples - finished.
        (⍺,nxt)∇ msk/⍵      ⍝ sieve remainder.
    }⍵{                     ⍝ from,
        (0=⍵|⍺)/⍵           ⍝ divisors of ⍵ in:
    }2,(1+2×⍳⌊0.5×⍵*÷2),⍵   ⍝ 2,3 5 .. sqrt(⍵),⍵
}

code_colours

test script

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