Ruddy Fur vs Dreadnought Yellow

How close are they?

ΔE 23.2, loose: different enough that you should pick on purpose. Dreadnought Yellow is noticeably lighter, much more saturated than Ruddy Fur.

Can I use Dreadnought Yellow instead of Ruddy Fur?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Dreadnought Yellow is noticeably lighter, much more saturated than Ruddy Fur.

Can I use Ruddy Fur instead of Dreadnought Yellow?

The same difference applies the other way round: Ruddy Fur is noticeably darker, 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.