Rotten Flesh vs Tyrant Skull

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Tyrant Skull is a little lighter, more muted than Rotten Flesh.

Can I use Tyrant Skull instead of Rotten Flesh?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Tyrant Skull is a little lighter, more muted than Rotten Flesh.

Can I use Rotten Flesh instead of Tyrant Skull?

The same difference applies the other way round: Rotten Flesh is a little darker, 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.