Tanker Grey vs Greatcoat Grey

How close are they?

ΔE 24.1, loose: different enough that you should pick on purpose. Greatcoat Grey is noticeably darker, warmer than Tanker Grey.

Can I use Greatcoat Grey instead of Tanker Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Greatcoat Grey is noticeably darker, warmer than Tanker Grey.

Can I use Tanker Grey instead of Greatcoat Grey?

The same difference applies the other way round: Tanker Grey is noticeably lighter, cooler.

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.