ΔE 28.9, loose: different enough that you should pick on purpose. Estus Yellow is noticeably lighter, more muted, cooler than Imperial Fist.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Estus Yellow is noticeably lighter, more muted, cooler than Imperial Fist.
The same difference applies the other way round: Imperial Fist is noticeably darker, more saturated, 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.
See Imperial Fist on the colour wheel See Estus Yellow on the colour wheel
All matches: Imperial Fist equivalents · Estus Yellow equivalents