Strong Grey vs IDF Blue

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. IDF Blue is more saturated, cooler than Strong Grey.

Can I use IDF Blue instead of Strong Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). IDF Blue is more saturated, cooler than Strong Grey.

Can I use Strong Grey instead of IDF Blue?

The same difference applies the other way round: Strong Grey is more muted, 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.