Red Opulence vs Martian Orange

How close are they?

ΔE 8.6, close: similar, with a visible difference side by side. Martian Orange is more muted than Red Opulence.

Can I use Martian Orange instead of Red Opulence?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Martian Orange is more muted than Red Opulence.

Can I use Red Opulence instead of Martian Orange?

The same difference applies the other way round: Red Opulence is 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.