Administratum Grey vs Trollblood Highlight

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Trollblood Highlight is noticeably lighter, more saturated than Administratum Grey.

Can I use Trollblood Highlight instead of Administratum Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Trollblood Highlight is noticeably lighter, more saturated than Administratum Grey.

Can I use Administratum Grey instead of Trollblood Highlight?

The same difference applies the other way round: Administratum Grey 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.