Sandstorm vs Flat Flesh

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Flat Flesh is a little darker, much more saturated, warmer than Sandstorm.

Can I use Flat Flesh instead of Sandstorm?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Flat Flesh is a little darker, much more saturated, warmer than Sandstorm.

Can I use Sandstorm instead of Flat Flesh?

The same difference applies the other way round: Sandstorm is a little lighter, much 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.