Shadow Grey vs IDF Blue

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. IDF Blue is a little lighter than Shadow Grey.

Can I use IDF Blue instead of Shadow Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). IDF Blue is a little lighter than Shadow Grey.

Can I use Shadow Grey instead of IDF Blue?

The same difference applies the other way round: Shadow Grey is a little darker.

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.