Dark Blue Tone vs Aether Blue

How close are they?

ΔE 33.8, loose: different enough that you should pick on purpose. Aether Blue is noticeably lighter, cooler than Dark Blue Tone.

Can I use Aether Blue instead of Dark Blue Tone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Aether Blue is noticeably lighter, cooler than Dark Blue Tone.

Can I use Dark Blue Tone instead of Aether Blue?

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