ΔE 18.8, loose: different enough that you should pick on purpose. Bright Green Blue is a little darker, much more saturated than Ulthuan Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Bright Green Blue is a little darker, much more saturated than Ulthuan Grey.
The same difference applies the other way round: Ulthuan Grey is a little lighter, much 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.
See Ulthuan Grey on the colour wheel See Bright Green Blue on the colour wheel
All matches: Ulthuan Grey equivalents · Bright Green Blue equivalents