Pastel Indigo vs Graphite Grey

How close are they?

ΔE 47.3, loose: different enough that you should pick on purpose. Graphite Grey is noticeably darker, much more muted than Pastel Indigo.

Can I use Graphite Grey instead of Pastel Indigo?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 47). Graphite Grey is noticeably darker, much more muted than Pastel Indigo.

Can I use Pastel Indigo instead of Graphite Grey?

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