ΔE 11.0, fair: the same colour family, but not a straight swap. Thunder Storm is a little darker, cooler than Dunkelgrau RAL 7021.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Thunder Storm is a little darker, cooler than Dunkelgrau RAL 7021.
The same difference applies the other way round: Dunkelgrau RAL 7021 is a little lighter, warmer.
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 Dunkelgrau RAL 7021 on the colour wheel See Thunder Storm on the colour wheel
All matches: Dunkelgrau RAL 7021 equivalents · Thunder Storm equivalents