Orc Flesh vs Greenskin

How close are they?

ΔE 9.8, close: similar, with a visible difference side by side. Greenskin is more saturated, cooler than Orc Flesh.

Can I use Greenskin instead of Orc Flesh?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Greenskin is more saturated, cooler than Orc Flesh.

Can I use Orc Flesh instead of Greenskin?

The same difference applies the other way round: Orc Flesh is more muted, 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.