Light Feldgrau vs Underhive Ash

How close are they?

ΔE 16.9, loose: different enough that you should pick on purpose. Underhive Ash is noticeably lighter, warmer than Light Feldgrau.

Can I use Underhive Ash instead of Light Feldgrau?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Underhive Ash is noticeably lighter, warmer than Light Feldgrau.

Can I use Light Feldgrau instead of Underhive Ash?

The same difference applies the other way round: Light Feldgrau is noticeably darker, 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.