Pistachio vs Moon Yellow

How close are they?

ΔE 20.8, loose: different enough that you should pick on purpose. Moon Yellow is a little lighter, more saturated, warmer than Pistachio.

Can I use Moon Yellow instead of Pistachio?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Moon Yellow is a little lighter, more saturated, warmer than Pistachio.

Can I use Pistachio instead of Moon Yellow?

The same difference applies the other way round: Pistachio is a little darker, more muted, 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.