C.P. Cadmium Yellow Dark vs Flat Yellow

How close are they?

ΔE 17.5, loose: different enough that you should pick on purpose. Flat Yellow is a little darker, more muted, warmer than C.P. Cadmium Yellow Dark.

Can I use Flat Yellow instead of C.P. Cadmium Yellow Dark?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Flat Yellow is a little darker, more muted, warmer than C.P. Cadmium Yellow Dark.

Can I use C.P. Cadmium Yellow Dark instead of Flat Yellow?

The same difference applies the other way round: C.P. Cadmium Yellow Dark is a little lighter, more saturated, 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.