Bad Bruise vs French Blue

How close are they?

ΔE 22.6, loose: different enough that you should pick on purpose. French Blue is noticeably lighter, much more saturated, cooler than Bad Bruise.

Can I use French Blue instead of Bad Bruise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). French Blue is noticeably lighter, much more saturated, cooler than Bad Bruise.

Can I use Bad Bruise instead of French Blue?

The same difference applies the other way round: Bad Bruise is noticeably darker, 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.