Elder Flower vs Red Purple

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Red Purple is a little darker, much more saturated, warmer than Elder Flower.

Can I use Red Purple instead of Elder Flower?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Red Purple is a little darker, much more saturated, warmer than Elder Flower.

Can I use Elder Flower instead of Red Purple?

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