Phantasmal Blue vs Trollblood Base

How close are they?

ΔE 20.9, loose: different enough that you should pick on purpose. Trollblood Base is noticeably darker, more muted, cooler than Phantasmal Blue.

Can I use Trollblood Base instead of Phantasmal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Trollblood Base is noticeably darker, more muted, cooler than Phantasmal Blue.

Can I use Phantasmal Blue instead of Trollblood Base?

The same difference applies the other way round: Phantasmal Blue is noticeably lighter, more saturated, 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.