Payne's Gray vs Ink Violet

How close are they?

ΔE 21.2, loose: different enough that you should pick on purpose. Ink Violet is a little darker, much more saturated, warmer than Payne's Gray.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Ink Violet is a little darker, much more saturated, warmer than Payne's Gray.

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

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

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.