Thunder Storm vs Slate Grey

How close are they?

ΔE 24.5, loose: different enough that you should pick on purpose. Slate Grey is noticeably lighter, more muted than Thunder Storm.

Can I use Slate Grey instead of Thunder Storm?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Slate Grey is noticeably lighter, more muted than Thunder Storm.

Can I use Thunder Storm instead of Slate Grey?

The same difference applies the other way round: Thunder Storm is noticeably darker, 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.