Red Copper vs Spartan Bronze

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Spartan Bronze is much more muted, warmer than Red Copper.

Can I use Spartan Bronze instead of Red Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Spartan Bronze is much more muted, warmer than Red Copper.

Can I use Red Copper instead of Spartan Bronze?

The same difference applies the other way round: Red Copper is much more saturated, 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.