ΔE 6.3, close: similar, with a visible difference side by side. Transparent Blue is a little darker than Altdorf Guard Blue. They are different kinds of paint: Citadel Altdorf Guard Blue is an opaque base/layer paint and Green Stuff World Transparent Blue is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Transparent Blue is a little darker than Altdorf Guard Blue. They are different kinds of paint.
The same difference applies the other way round: Altdorf Guard Blue is a little lighter.
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.
See Altdorf Guard Blue on the colour wheel See Transparent Blue on the colour wheel
All matches: Altdorf Guard Blue equivalents · Transparent Blue equivalents