Trollblood Base vs Light Gray

How close are they?

ΔE 21.3, loose: different enough that you should pick on purpose. Light Gray is a little lighter, much more muted than Trollblood Base.

Can I use Light Gray instead of Trollblood Base?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Light Gray is a little lighter, much more muted than Trollblood Base.

Can I use Trollblood Base instead of Light Gray?

The same difference applies the other way round: Trollblood Base is a little darker, much 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.