Kantor Blue vs Indigo Sky

How close are they?

ΔE 20.0, loose: different enough that you should pick on purpose. Indigo Sky is noticeably lighter, more muted than Kantor Blue.

Can I use Indigo Sky instead of Kantor Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Indigo Sky is noticeably lighter, more muted than Kantor Blue.

Can I use Kantor Blue instead of Indigo Sky?

The same difference applies the other way round: Kantor Blue is noticeably darker, more saturated.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.