Sekigahara vs Vermilion

How close are they?

ΔE 18.4, loose: different enough that you should pick on purpose. Vermilion is a little darker, much more saturated than Sekigahara.

Can I use Vermilion instead of Sekigahara?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Vermilion is a little darker, much more saturated than Sekigahara.

Can I use Sekigahara instead of Vermilion?

The same difference applies the other way round: Sekigahara is a little lighter, much 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.