Tyrant Skull vs Titan Mars Pale

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Titan Mars Pale is a little lighter, warmer than Tyrant Skull.

Can I use Titan Mars Pale instead of Tyrant Skull?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Titan Mars Pale is a little lighter, warmer than Tyrant Skull.

Can I use Tyrant Skull instead of Titan Mars Pale?

The same difference applies the other way round: Tyrant Skull is a little darker, cooler.

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.