Pastel Indigo vs Dark Green Blue

How close are they?

ΔE 48.6, loose: different enough that you should pick on purpose. Dark Green Blue is noticeably darker, more muted than Pastel Indigo.

Can I use Dark Green Blue instead of Pastel Indigo?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 49). Dark Green Blue is noticeably darker, more muted than Pastel Indigo.

Can I use Pastel Indigo instead of Dark Green Blue?

The same difference applies the other way round: Pastel Indigo is noticeably lighter, 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.