Yriel Yellow vs Sun Yellow

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Sun Yellow is a little lighter, more saturated, cooler than Yriel Yellow.

Can I use Sun Yellow instead of Yriel Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Sun Yellow is a little lighter, more saturated, cooler than Yriel Yellow.

Can I use Yriel Yellow instead of Sun Yellow?

The same difference applies the other way round: Yriel Yellow is a little darker, 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.