Fiendish Yellow vs Khemri Sands

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Khemri Sands is noticeably darker, more muted than Fiendish Yellow.

Can I use Khemri Sands instead of Fiendish Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Khemri Sands is noticeably darker, more muted than Fiendish Yellow.

Can I use Fiendish Yellow instead of Khemri Sands?

The same difference applies the other way round: Fiendish Yellow is noticeably lighter, more saturated.

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.