Dark Blue Black vs Gloomy Violet

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Gloomy Violet is a little lighter, much more saturated, warmer than Dark Blue Black.

Can I use Gloomy Violet instead of Dark Blue Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Gloomy Violet is a little lighter, much more saturated, warmer than Dark Blue Black.

Can I use Dark Blue Black instead of Gloomy Violet?

The same difference applies the other way round: Dark Blue Black is a little darker, 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.