It's opaque white glass, cased in clear and the red spiral added on top of that. The casing is not
inbetween the red bits, it's under them.
Unless the white is created by adding powdered white enamel to a clear gather.
White is a bit of an awkward colour in glass - it tends to have a different melting point to other colours and can be "incompatible", creating problems. I think it is quite common to use coloured powder on a clear gather to give a base colour, rather than melt a whole pot of coloured glass.