Pastel Blue vs Forest Faun

How close are they?

ΔE 6.7, close: similar, with a visible difference side by side. Forest Faun is a little darker than Pastel Blue.

Can I use Forest Faun instead of Pastel Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Forest Faun is a little darker than Pastel Blue.

Can I use Pastel Blue instead of Forest Faun?

The same difference applies the other way round: Pastel 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.