Knight-Questor Flesh vs Red FS11136

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Red FS11136 is much more saturated, cooler than Knight-Questor Flesh.

Can I use Red FS11136 instead of Knight-Questor Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Red FS11136 is much more saturated, cooler than Knight-Questor Flesh.

Can I use Knight-Questor Flesh instead of Red FS11136?

The same difference applies the other way round: Knight-Questor Flesh is much more muted, 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.