Inktense Brown vs Ink Blue

How close are they?

ΔE 37.6, loose: different enough that you should pick on purpose. Ink Blue is a little lighter, much more saturated than Inktense Brown.

Can I use Ink Blue instead of Inktense Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Ink Blue is a little lighter, much more saturated than Inktense Brown.

Can I use Inktense Brown instead of Ink Blue?

The same difference applies the other way round: Inktense Brown is a little darker, much more muted.

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.