Castellan Green vs Spring Green

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Spring Green is much more saturated, warmer than Castellan Green.

Can I use Spring Green instead of Castellan Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Spring Green is much more saturated, warmer than Castellan Green.

Can I use Castellan Green instead of Spring Green?

The same difference applies the other way round: Castellan Green is 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.