Brown Ink vs Ink Dark Turquoise

How close are they?

ΔE 34.5, loose: different enough that you should pick on purpose. Ink Dark Turquoise is more muted, cooler than Brown Ink.

Can I use Ink Dark Turquoise instead of Brown Ink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Ink Dark Turquoise is more muted, cooler than Brown Ink.

Can I use Brown Ink instead of Ink Dark Turquoise?

The same difference applies the other way round: Brown Ink is more saturated, 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.