Royal Cloak vs Cobalt Bluegreen

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Cobalt Bluegreen is noticeably lighter, more muted than Royal Cloak.

Can I use Cobalt Bluegreen instead of Royal Cloak?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Cobalt Bluegreen is noticeably lighter, more muted than Royal Cloak.

Can I use Royal Cloak instead of Cobalt Bluegreen?

The same difference applies the other way round: Royal Cloak is noticeably darker, more saturated.

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.