Kantor Blue vs Blue Violet

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Blue Violet is noticeably lighter, warmer than Kantor Blue.

Can I use Blue Violet instead of Kantor Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Blue Violet is noticeably lighter, warmer than Kantor Blue.

Can I use Kantor Blue instead of Blue Violet?

The same difference applies the other way round: Kantor Blue is noticeably darker, 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.