Algae Green vs Olive Yellow

How close are they?

ΔE 21.7, loose: different enough that you should pick on purpose. Olive Yellow is much more saturated, warmer than Algae Green.

Can I use Olive Yellow instead of Algae Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Olive Yellow is much more saturated, warmer than Algae Green.

Can I use Algae Green instead of Olive Yellow?

The same difference applies the other way round: Algae 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.