Heartfire vs Light Yellow

How close are they?

ΔE 25.0, loose: different enough that you should pick on purpose. Light Yellow is a little lighter, more saturated, cooler than Heartfire.

Can I use Light Yellow instead of Heartfire?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Light Yellow is a little lighter, more saturated, cooler than Heartfire.

Can I use Heartfire instead of Light Yellow?

The same difference applies the other way round: Heartfire is a little darker, more muted, 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.