Viridian Grim vs Splinter Green

How close are they?

ΔE 21.6, loose: different enough that you should pick on purpose. Splinter Green is noticeably lighter, much more saturated than Viridian Grim.

Can I use Splinter Green instead of Viridian Grim?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Splinter Green is noticeably lighter, much more saturated than Viridian Grim.

Can I use Viridian Grim instead of Splinter Green?

The same difference applies the other way round: Viridian Grim is noticeably darker, much 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.