Foundry Red vs Gemstone

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Gemstone is noticeably darker, more muted than Foundry Red.

Can I use Gemstone instead of Foundry Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Gemstone is noticeably darker, more muted than Foundry Red.

Can I use Foundry Red instead of Gemstone?

The same difference applies the other way round: Foundry Red is noticeably lighter, 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.