Royal Robes vs Deep Blue

How close are they?

ΔE 24.0, loose: different enough that you should pick on purpose. Deep Blue is noticeably darker, much more saturated, warmer than Royal Robes.

Can I use Deep Blue instead of Royal Robes?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Deep Blue is noticeably darker, much more saturated, warmer than Royal Robes.

Can I use Royal Robes instead of Deep Blue?

The same difference applies the other way round: Royal Robes is noticeably lighter, much more muted, 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.