Gothic Blue vs Eldar Robe

How close are they?

ΔE 17.4, loose: different enough that you should pick on purpose. Eldar Robe is noticeably lighter, more saturated, cooler than Gothic Blue.

Can I use Eldar Robe instead of Gothic Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Eldar Robe is noticeably lighter, more saturated, cooler than Gothic Blue.

Can I use Gothic Blue instead of Eldar Robe?

The same difference applies the other way round: Gothic Blue is noticeably darker, 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.