Titan Green Pale vs Desert Tan

How close are they?

ΔE 21.6, loose: different enough that you should pick on purpose. Desert Tan is a little darker, much more saturated, warmer than Titan Green Pale.

Can I use Desert Tan instead of Titan Green Pale?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Desert Tan is a little darker, much more saturated, warmer than Titan Green Pale.

Can I use Titan Green Pale instead of Desert Tan?

The same difference applies the other way round: Titan Green Pale 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.