Dark Blue Tone vs Ultramarine Shadow

How close are they?

ΔE 13.7, fair: the same colour family, but not a straight swap. Ultramarine Shadow is more muted, cooler than Dark Blue Tone. They are different kinds of paint: Army Painter Dark Blue Tone is a wash/shade and Reaper Ultramarine Shadow 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 Ultramarine Shadow instead of Dark Blue Tone?

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Ultramarine Shadow is more muted, cooler than Dark Blue Tone. They are different kinds of paint.

Can I use Dark Blue Tone instead of Ultramarine Shadow?

The same difference applies the other way round: Dark Blue Tone is more saturated, 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.