Kantor Blue vs Cygnar Blue Base

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. Cygnar Blue Base is noticeably lighter, more saturated than Kantor Blue.

Can I use Cygnar Blue Base instead of Kantor Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Cygnar Blue Base is noticeably lighter, more saturated than Kantor Blue.

Can I use Kantor Blue instead of Cygnar Blue Base?

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