Phthalo Blue (Red Shade) vs Ink Blue

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Ink Blue is noticeably lighter, more saturated, cooler than Phthalo Blue (Red Shade).

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ink Blue is noticeably lighter, more saturated, cooler than Phthalo Blue (Red Shade).

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

The same difference applies the other way round: Phthalo Blue (Red Shade) is noticeably darker, more muted, warmer.

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.