Brown Ink vs Smoke Ink

How close are they?

ΔE 21.8, loose: different enough that you should pick on purpose. Smoke Ink is noticeably darker, much more muted than Brown Ink.

Can I use Smoke Ink instead of Brown Ink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Smoke Ink is noticeably darker, much more muted than Brown Ink.

Can I use Brown Ink instead of Smoke Ink?

The same difference applies the other way round: Brown Ink is noticeably lighter, much 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.