ΔE 13.1, fair: the same colour family, but not a straight swap. Rust (Metallic) is noticeably lighter than Mythril Blade.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Rust (Metallic) is noticeably lighter than Mythril Blade.
The same difference applies the other way round: Mythril Blade is noticeably darker.
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 Mythril Blade on the colour wheel See Rust (Metallic) on the colour wheel
All matches: Mythril Blade equivalents · Rust (Metallic) equivalents