Naphthol Red Medium vs Ink Yellow

How close are they?

ΔE 54.1, loose: different enough that you should pick on purpose. Ink Yellow is noticeably lighter, warmer than Naphthol Red Medium.

Can I use Ink Yellow instead of Naphthol Red Medium?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 54). Ink Yellow is noticeably lighter, warmer than Naphthol Red Medium.

Can I use Naphthol Red Medium instead of Ink Yellow?

The same difference applies the other way round: Naphthol Red Medium is noticeably darker, 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.