Violent Vermillion vs Trollslayer Orange

How close are they?

ΔE 21.9, loose: different enough that you should pick on purpose. Trollslayer Orange is much more saturated, warmer than Violent Vermillion.

Can I use Trollslayer Orange instead of Violent Vermillion?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Trollslayer Orange is much more saturated, warmer than Violent Vermillion.

Can I use Violent Vermillion instead of Trollslayer Orange?

The same difference applies the other way round: Violent Vermillion is 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.