ΔE 28.7, loose: different enough that you should pick on purpose. Undead Dragon is a little lighter, much more muted, warmer than Pastel Indigo.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Undead Dragon is a little lighter, much more muted, warmer than Pastel Indigo.
The same difference applies the other way round: Pastel Indigo is a little darker, much more saturated, 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 Pastel Indigo on the colour wheel See Undead Dragon on the colour wheel
All matches: Pastel Indigo equivalents · Undead Dragon equivalents