IDF Sand Grey 1970s-1980s vs Green Yellow

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Green Yellow is cooler than IDF Sand Grey 1970s-1980s.

Can I use Green Yellow instead of IDF Sand Grey 1970s-1980s?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Green Yellow is cooler than IDF Sand Grey 1970s-1980s.

Can I use IDF Sand Grey 1970s-1980s instead of Green Yellow?

The same difference applies the other way round: IDF Sand Grey 1970s-1980s is 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.