ΔE 16.6, loose: different enough that you should pick on purpose. Barley Gray BS4800/18B21 is a little darker, more saturated, warmer than Celestra Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Barley Gray BS4800/18B21 is a little darker, more saturated, warmer than Celestra Grey.
The same difference applies the other way round: Celestra Grey is a little lighter, more muted, cooler.
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.
See Celestra Grey on the colour wheel See Barley Gray BS4800/18B21 on the colour wheel
All matches: Celestra Grey equivalents · Barley Gray BS4800/18B21 equivalents