Autumn Sage vs Bright Green Blue

How close are they?

ΔE 8.9, close: similar, with a visible difference side by side. Bright Green Blue is a little lighter, cooler than Autumn Sage.

Can I use Bright Green Blue instead of Autumn Sage?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Bright Green Blue is a little lighter, cooler than Autumn Sage.

Can I use Autumn Sage instead of Bright Green Blue?

The same difference applies the other way round: Autumn Sage is a little darker, 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.