Hoplite Gold vs Green Sun

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Green Sun is noticeably lighter, more muted than Hoplite Gold.

Can I use Green Sun instead of Hoplite Gold?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Green Sun is noticeably lighter, more muted than Hoplite Gold.

Can I use Hoplite Gold instead of Green Sun?

The same difference applies the other way round: Hoplite Gold is noticeably darker, 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.