2 different recipes. Brands sell different pigment mixtures under this name, so they won't all look or mix alike. Compare the pigments below.
Who makes it
PB35 Cerulean Blue1
- Williamsburg Handmade Oil Colors No. 6000848
PB36 Cobalt Turquoise / Cerulean1
- Da Vinci Artists' Oil Color Cerulean Blue Genuine
Sizes are the ones each line is sold in; marks a size only some colors come in. Check the store for a particular color.