Hierotek Green vs Green Sky

How close are they?

ΔE 27.5, loose: different enough that you should pick on purpose. Green Sky is a little darker, much more muted, warmer than Hierotek Green.

Can I use Green Sky instead of Hierotek Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Green Sky is a little darker, much more muted, warmer than Hierotek Green.

Can I use Hierotek Green instead of Green Sky?

The same difference applies the other way round: Hierotek Green is a little lighter, much more saturated, cooler.

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.