Troglodyte Blue vs Ironhull Grey

How close are they?

ΔE 29.3, loose: different enough that you should pick on purpose. Ironhull Grey is a little lighter, much more muted than Troglodyte Blue.

Can I use Ironhull Grey instead of Troglodyte Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Ironhull Grey is a little lighter, much more muted than Troglodyte Blue.

Can I use Troglodyte Blue instead of Ironhull Grey?

The same difference applies the other way round: Troglodyte Blue 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.