ΔE 12.8, fair: the same colour family, but not a straight swap. Frost Blue is noticeably darker, more saturated, warmer than WWI German Light Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Frost Blue is noticeably darker, more saturated, warmer than WWI German Light Blue.
The same difference applies the other way round: WWI German Light Blue is noticeably 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 WWI German Light Blue on the colour wheel See Frost Blue on the colour wheel
All matches: WWI German Light Blue equivalents · Frost Blue equivalents