RLM 02 RLM-Grau (1938) vs Underhive Ash

How close are they?

ΔE 7.7, close: similar, with a visible difference side by side. Underhive Ash is a little lighter, cooler than RLM 02 RLM-Grau (1938).

Can I use Underhive Ash instead of RLM 02 RLM-Grau (1938)?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Underhive Ash is a little lighter, cooler than RLM 02 RLM-Grau (1938).

Can I use RLM 02 RLM-Grau (1938) instead of Underhive Ash?

The same difference applies the other way round: RLM 02 RLM-Grau (1938) is a little darker, 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.