Violent Vermillion vs Khador Red Highlight

How close are they?

ΔE 24.3, loose: different enough that you should pick on purpose. Khador Red Highlight is more saturated, warmer than Violent Vermillion.

Can I use Khador Red Highlight instead of Violent Vermillion?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Khador Red Highlight is more saturated, warmer than Violent Vermillion.

Can I use Violent Vermillion instead of Khador Red Highlight?

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