Royal Cloak vs Eldritch

How close are they?

ΔE 28.8, loose: different enough that you should pick on purpose. Eldritch is noticeably darker, warmer than Royal Cloak.

Can I use Eldritch instead of Royal Cloak?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Eldritch is noticeably darker, warmer than Royal Cloak.

Can I use Royal Cloak instead of Eldritch?

The same difference applies the other way round: Royal Cloak is noticeably lighter, cooler.

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.