Blue Gorkshroom vs Blue

How close are they?

ΔE 16.7, loose: different enough that you should pick on purpose. Blue is noticeably lighter, more muted than Blue Gorkshroom.

Can I use Blue instead of Blue Gorkshroom?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Blue is noticeably lighter, more muted than Blue Gorkshroom.

Can I use Blue Gorkshroom instead of Blue?

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