Mythical Orange vs Demon Red

How close are they?

ΔE 22.4, loose: different enough that you should pick on purpose. Demon Red is a little darker, much more muted, cooler than Mythical Orange.

Can I use Demon Red instead of Mythical Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Demon Red is a little darker, much more muted, cooler than Mythical Orange.

Can I use Mythical Orange instead of Demon Red?

The same difference applies the other way round: Mythical Orange is a little lighter, much more saturated, 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.