Valkyrie Yellow vs Sand Yellow

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Sand Yellow is a little darker, warmer than Valkyrie Yellow.

Can I use Sand Yellow instead of Valkyrie Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Sand Yellow is a little darker, warmer than Valkyrie Yellow.

Can I use Valkyrie Yellow instead of Sand Yellow?

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