Magmadroth Flame vs Red Opulence

How close are they?

ΔE 26.1, loose: different enough that you should pick on purpose. Red Opulence is noticeably darker, cooler than Magmadroth Flame.

Can I use Red Opulence instead of Magmadroth Flame?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Red Opulence is noticeably darker, cooler than Magmadroth Flame.

Can I use Magmadroth Flame instead of Red Opulence?

The same difference applies the other way round: Magmadroth Flame is noticeably lighter, warmer.

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.