Pylar Glacier vs Graphite Grey

How close are they?

ΔE 33.4, loose: different enough that you should pick on purpose. Graphite Grey is noticeably darker, more muted than Pylar Glacier.

Can I use Graphite Grey instead of Pylar Glacier?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 33). Graphite Grey is noticeably darker, more muted than Pylar Glacier.

Can I use Pylar Glacier instead of Graphite Grey?

The same difference applies the other way round: Pylar Glacier is noticeably lighter, 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.