Phthalo Blue (Red Shade) vs Ink Black

How close are they?

ΔE 27.4, loose: different enough that you should pick on purpose. Ink Black is a little darker, much more muted than Phthalo Blue (Red Shade).

Can I use Ink Black instead of Phthalo Blue (Red Shade)?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Ink Black is a little darker, much more muted than Phthalo Blue (Red Shade).

Can I use Phthalo Blue (Red Shade) instead of Ink Black?

The same difference applies the other way round: Phthalo Blue (Red Shade) is a little lighter, much 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.