Bismuth Yellow vs Deep Yellow

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Deep Yellow is a little darker than Bismuth Yellow.

Can I use Deep Yellow instead of Bismuth Yellow?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Deep Yellow is a little darker than Bismuth Yellow.

Can I use Bismuth Yellow instead of Deep Yellow?

The same difference applies the other way round: Bismuth Yellow is a little lighter.

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.