ΔE 6.5, close: similar, with a visible difference side by side. Royal Purple is a little darker than Dark Violet.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Royal Purple is a little darker than Dark Violet.
The same difference applies the other way round: Dark Violet is a little lighter.
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.
See Dark Violet on the colour wheel See Royal Purple on the colour wheel
All matches: Dark Violet equivalents · Royal Purple equivalents