Trollblood Highlight vs Pale Blue Grey

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Pale Blue Grey is noticeably darker, more muted than Trollblood Highlight.

Can I use Pale Blue Grey instead of Trollblood Highlight?

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

Can I use Trollblood Highlight instead of Pale Blue Grey?

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

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.