Pumpkin vs Tan

How close are they?

ΔE 18.6, loose: different enough that you should pick on purpose. Tan is a little darker, much more muted than Pumpkin.

Can I use Tan instead of Pumpkin?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Tan is a little darker, much more muted than Pumpkin.

Can I use Pumpkin instead of Tan?

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

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.