Squig Orange vs Quinacridone Red Light

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Quinacridone Red Light is noticeably darker, much more saturated than Squig Orange.

Can I use Quinacridone Red Light instead of Squig Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Quinacridone Red Light is noticeably darker, much more saturated than Squig Orange.

Can I use Squig Orange instead of Quinacridone Red Light?

The same difference applies the other way round: Squig Orange is noticeably lighter, much 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.