Nickel Azo Yellow vs Ink Yellow

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Ink Yellow is a little darker, warmer than Nickel Azo Yellow.

Can I use Ink Yellow instead of Nickel Azo Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ink Yellow is a little darker, warmer than Nickel Azo Yellow.

Can I use Nickel Azo Yellow instead of Ink Yellow?

The same difference applies the other way round: Nickel Azo Yellow is a little lighter, 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.