Slate Grey vs Bleached Strap Grey

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Bleached Strap Grey is a little darker than Slate Grey.

Can I use Bleached Strap Grey instead of Slate Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Bleached Strap Grey is a little darker than Slate Grey.

Can I use Slate Grey instead of Bleached Strap Grey?

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

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.