Dechala Lilac vs Faded Ultramarine

How close are they?

ΔE 21.3, loose: different enough that you should pick on purpose. Faded Ultramarine is a little darker, cooler than Dechala Lilac.

Can I use Faded Ultramarine instead of Dechala Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Faded Ultramarine is a little darker, cooler than Dechala Lilac.

Can I use Dechala Lilac instead of Faded Ultramarine?

The same difference applies the other way round: Dechala Lilac is a little 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.