ΔE 17.8, loose: different enough that you should pick on purpose. Signal Red (Metal.) is noticeably lighter, more saturated than Mithril.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Signal Red (Metal.) is noticeably lighter, more saturated than Mithril.
The same difference applies the other way round: Mithril is noticeably darker, more muted.
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 Mithril on the colour wheel See Signal Red (Metal.) on the colour wheel
All matches: Mithril equivalents · Signal Red (Metal.) equivalents