Deep Blue vs Flat Blue

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Flat Blue is noticeably lighter, more saturated, cooler than Deep Blue.

Can I use Flat Blue instead of Deep Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Flat Blue is noticeably lighter, more saturated, cooler than Deep Blue.

Can I use Deep Blue instead of Flat Blue?

The same difference applies the other way round: Deep Blue is noticeably darker, 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.