Solar Sand vs Ancient Copper

How close are they?

ΔE 21.0, loose: different enough that you should pick on purpose. Ancient Copper is noticeably darker, warmer than Solar Sand.

Can I use Ancient Copper instead of Solar Sand?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Ancient Copper is noticeably darker, warmer than Solar Sand.

Can I use Solar Sand instead of Ancient Copper?

The same difference applies the other way round: Solar Sand is noticeably lighter, 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.