Mage Hunter Green vs Field Blue

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Field Blue is a little darker, more muted than Mage Hunter Green.

Can I use Field Blue instead of Mage Hunter Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Field Blue is a little darker, more muted than Mage Hunter Green.

Can I use Mage Hunter Green instead of Field Blue?

The same difference applies the other way round: Mage Hunter Green is a little lighter, more saturated.

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.