ΔE 17.4, loose: different enough that you should pick on purpose. Indigo is a little darker, much more saturated, warmer than Deep Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Indigo is a little darker, much more saturated, warmer than Deep Grey.
The same difference applies the other way round: Deep Grey 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.
See Deep Grey on the colour wheel See Indigo on the colour wheel
All matches: Deep Grey equivalents · Indigo equivalents