ΔE 18.4, loose: different enough that you should pick on purpose. C.P. Cadmium Yellow Dark is much more saturated than Phoenix Flames.
Can I use C.P. Cadmium Yellow Dark instead of Phoenix Flames?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). C.P. Cadmium Yellow Dark is much more saturated than Phoenix Flames.
Can I use Phoenix Flames instead of C.P. Cadmium Yellow Dark?
The same difference applies the other way round: Phoenix Flames is much more muted.
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.