Dark Reaper vs Dragon Blue

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Dragon Blue is a little lighter, more saturated, warmer than Dark Reaper.

Can I use Dragon Blue instead of Dark Reaper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Dragon Blue is a little lighter, more saturated, warmer than Dark Reaper.

Can I use Dark Reaper instead of Dragon Blue?

The same difference applies the other way round: Dark Reaper is a little darker, more muted, cooler.

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.