Payne's Gray vs Ink Blue

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Ink Blue is a little darker, much more saturated than Payne's Gray.

Can I use Ink Blue instead of Payne's Gray?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Ink Blue is a little darker, much more saturated than Payne's Gray.

Can I use Payne's Gray instead of Ink Blue?

The same difference applies the other way round: Payne's Gray is a little lighter, 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.