Desert Skin vs Mars Yellow

How close are they?

ΔE 16.4, loose: different enough that you should pick on purpose. Mars Yellow is a little darker, much more saturated, cooler than Desert Skin.

Can I use Mars Yellow instead of Desert Skin?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Mars Yellow is a little darker, much more saturated, cooler than Desert Skin.

Can I use Desert Skin instead of Mars Yellow?

The same difference applies the other way round: Desert Skin is a little lighter, much more muted, 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.