Topaz Skin vs Mars Yellow

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Mars Yellow is more saturated, warmer than Topaz Skin.

Can I use Mars Yellow instead of Topaz Skin?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Mars Yellow is more saturated, warmer than Topaz Skin.

Can I use Topaz Skin instead of Mars Yellow?

The same difference applies the other way round: Topaz Skin is more muted, 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.