Elder Flower vs Chestnut Brown

How close are they?

ΔE 27.7, loose: different enough that you should pick on purpose. Chestnut Brown is noticeably darker, much more muted, warmer than Elder Flower.

Can I use Chestnut Brown instead of Elder Flower?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Chestnut Brown is noticeably darker, much more muted, warmer than Elder Flower.

Can I use Elder Flower instead of Chestnut Brown?

The same difference applies the other way round: Elder Flower is noticeably lighter, much 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.