Dark Blue Tone vs Ritterlich Blue

How close are they?

ΔE 12.3, fair: the same colour family, but not a straight swap. Ritterlich Blue is a little lighter, cooler than Dark Blue Tone. They are different kinds of paint: Army Painter Dark Blue Tone is a wash/shade and Reaper Ritterlich Blue is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.

Can I use Ritterlich Blue instead of Dark Blue Tone?

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Ritterlich Blue is a little lighter, cooler than Dark Blue Tone. They are different kinds of paint.

Can I use Dark Blue Tone instead of Ritterlich Blue?

The same difference applies the other way round: Dark Blue Tone is a little 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.