Royal Robes vs Dark Purple

How close are they?

ΔE 32.6, loose: different enough that you should pick on purpose. Dark Purple is noticeably darker, warmer than Royal Robes.

Can I use Dark Purple instead of Royal Robes?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 33). Dark Purple is noticeably darker, warmer than Royal Robes.

Can I use Royal Robes instead of Dark Purple?

The same difference applies the other way round: Royal Robes 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.