Pastel Lavender vs Ancestral Blue

How close are they?

ΔE 33.8, loose: different enough that you should pick on purpose. Ancestral Blue is noticeably darker, cooler than Pastel Lavender.

Can I use Ancestral Blue instead of Pastel Lavender?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Ancestral Blue is noticeably darker, cooler than Pastel Lavender.

Can I use Pastel Lavender instead of Ancestral Blue?

The same difference applies the other way round: Pastel Lavender is noticeably lighter, 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.