Imperial Fist vs Imperial Yellow

How close are they?

ΔE 16.9, loose: different enough that you should pick on purpose. Imperial Yellow is a little darker, warmer than Imperial Fist.

Can I use Imperial Yellow instead of Imperial Fist?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Imperial Yellow is a little darker, warmer than Imperial Fist.

Can I use Imperial Fist instead of Imperial Yellow?

The same difference applies the other way round: Imperial Fist 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.