Orange vs Dark Vermilion

How close are they?

ΔE 9.9, close: similar, with a visible difference side by side. Dark Vermilion is a little darker, more saturated than Orange.

Can I use Dark Vermilion instead of Orange?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Dark Vermilion is a little darker, more saturated than Orange.

Can I use Orange instead of Dark Vermilion?

The same difference applies the other way round: Orange is a little lighter, more muted.

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.