Neon Yellow vs Red Ecstasy

How close are they?

ΔE 106.1, loose: different enough that you should pick on purpose. Red Ecstasy is noticeably darker, much more muted, warmer than Neon Yellow.

Can I use Red Ecstasy instead of Neon Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 106). Red Ecstasy is noticeably darker, much more muted, warmer than Neon Yellow.

Can I use Neon Yellow instead of Red Ecstasy?

The same difference applies the other way round: Neon Yellow is noticeably lighter, much more saturated, 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.