Rlyeh Grey vs Light Grey RLM63

How close are they?

ΔE 10.7, fair: the same colour family, but not a straight swap. Light Grey RLM63 is a little lighter, more saturated than Rlyeh Grey.

Can I use Light Grey RLM63 instead of Rlyeh Grey?

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Light Grey RLM63 is a little lighter, more saturated than Rlyeh Grey.

Can I use Rlyeh Grey instead of Light Grey RLM63?

The same difference applies the other way round: Rlyeh Grey is a little darker, 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.