Khador Red Highlight vs Yellow RLM04

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Yellow RLM04 is a little lighter, cooler than Khador Red Highlight.

Can I use Yellow RLM04 instead of Khador Red Highlight?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Yellow RLM04 is a little lighter, cooler than Khador Red Highlight.

Can I use Khador Red Highlight instead of Yellow RLM04?

The same difference applies the other way round: Khador Red Highlight is a little darker, warmer.

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.