2 different recipes. Brands sell different pigment mixtures under this name, so they won't all look or mix alike. Compare the pigments below; the most common is PB28 Cobalt Blue.
Who makes it
PB28 Cobalt Blue2
- Rembrandt Artists' Oil Colour No. 513
- Schmincke Mussini Resin-Oil No. 10480
PB36 Cobalt Turquoise / Cerulean1
- Maimeri Puro Oil Colours No. 373
Sizes are the ones each line is sold in; marks a size only some colors come in. Check the store for a particular color.