Elder Flower vs Light Pink

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Light Pink is noticeably lighter, more saturated than Elder Flower.

Can I use Light Pink instead of Elder Flower?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Light Pink is noticeably lighter, more saturated than Elder Flower.

Can I use Elder Flower instead of Light Pink?

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