Yellow Flame vs Sun Yellow

How close are they?

ΔE 19.2, loose: different enough that you should pick on purpose. Sun Yellow is a little darker, more saturated, warmer than Yellow Flame.

Can I use Sun Yellow instead of Yellow Flame?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Sun Yellow is a little darker, more saturated, warmer than Yellow Flame.

Can I use Yellow Flame instead of Sun Yellow?

The same difference applies the other way round: Yellow Flame is a little lighter, 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.