Terracotta Pie vs US Earth Yellow

How close are they?

ΔE 22.6, loose: different enough that you should pick on purpose. US Earth Yellow is a little darker, much more muted than Terracotta Pie.

Can I use US Earth Yellow instead of Terracotta Pie?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). US Earth Yellow is a little darker, much more muted than Terracotta Pie.

Can I use Terracotta Pie instead of US Earth Yellow?

The same difference applies the other way round: Terracotta Pie is a little lighter, much 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.