Grimdark Shadow vs Targor Rageshade

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Targor Rageshade is a little lighter, more saturated than Grimdark Shadow.

Can I use Targor Rageshade instead of Grimdark Shadow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Targor Rageshade is a little lighter, more saturated than Grimdark Shadow.

Can I use Grimdark Shadow instead of Targor Rageshade?

The same difference applies the other way round: Grimdark Shadow is a little 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.