Red viscosity identification???

[Jwitt52]

I’ve noticed while oil shopping some oil typically with higher viscosity ratings have red numbers and letter. Why are these numbers in red? Like Castrol 5w-50 or Mobil 15w-50 and a few others.

 

[RooflessVW]

Differentiation. If you look at Mobil/Castrol containers each visco has a different print/background color.