Hexed Violet vs Indigo Sky

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Indigo Sky is noticeably darker, cooler than Hexed Violet.

Can I use Indigo Sky instead of Hexed Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Indigo Sky is noticeably darker, cooler than Hexed Violet.

Can I use Hexed Violet instead of Indigo Sky?

The same difference applies the other way round: Hexed Violet is noticeably lighter, warmer.

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.