ΔE 45.1, loose: different enough that you should pick on purpose. Space Grey is noticeably darker, more saturated, cooler than Undead Dragon.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 45). Space Grey is noticeably darker, more saturated, cooler than Undead Dragon.
The same difference applies the other way round: Undead Dragon is noticeably lighter, more muted, warmer.
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 Undead Dragon on the colour wheel See Space Grey on the colour wheel
All matches: Undead Dragon equivalents · Space Grey equivalents