Putrid White vs Dirty Grey

How close are they?

ΔE 7.8, close: similar, with a visible difference side by side. Dirty Grey is a little darker, more saturated than Putrid White.

Can I use Dirty Grey instead of Putrid White?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Dirty Grey is a little darker, more saturated than Putrid White.

Can I use Putrid White instead of Dirty Grey?

The same difference applies the other way round: Putrid White is a little lighter, 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.