Dark Grey vs Thunder Storm

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Thunder Storm is a little darker, more saturated than Dark Grey.

Can I use Thunder Storm instead of Dark Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Thunder Storm is a little darker, more saturated than Dark Grey.

Can I use Dark Grey instead of Thunder Storm?

The same difference applies the other way round: Dark Grey is a little lighter, 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.