Heartfire vs Yellow RLM04

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Yellow RLM04 is a little darker than Heartfire.

Can I use Yellow RLM04 instead of Heartfire?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Yellow RLM04 is a little darker than Heartfire.

Can I use Heartfire instead of Yellow RLM04?

The same difference applies the other way round: Heartfire is a little lighter.

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.