Moldy Ochre vs Lemon Yellow

How close are they?

ΔE 21.8, loose: different enough that you should pick on purpose. Lemon Yellow is a little lighter, much more saturated, cooler than Moldy Ochre.

Can I use Lemon Yellow instead of Moldy Ochre?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Lemon Yellow is a little lighter, much more saturated, cooler than Moldy Ochre.

Can I use Moldy Ochre instead of Lemon Yellow?

The same difference applies the other way round: Moldy Ochre is a little darker, much more muted, warmer.

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.