ΔE 20.3, loose: different enough that you should pick on purpose. Knight-Questor Flesh is much more saturated than Black Rose Immortal.
Can I use Knight-Questor Flesh instead of Black Rose Immortal?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Knight-Questor Flesh is much more saturated than Black Rose Immortal.
Can I use Black Rose Immortal instead of Knight-Questor Flesh?
The same difference applies the other way round: Black Rose Immortal is much more muted.
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.