Matt White vs Ion Blue

How close are they?

ΔE 19.8, loose: different enough that you should pick on purpose. Ion Blue is noticeably darker, more saturated than Matt White.

Can I use Ion Blue instead of Matt White?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ion Blue is noticeably darker, more saturated than Matt White.

Can I use Matt White instead of Ion Blue?

The same difference applies the other way round: Matt White is noticeably lighter, more muted.

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.