It's very variable, Connie. I have seen authentic pieces with clear 'swirls', but I regard this as a 'flaw'.
Generally the glass has a consistent colour, and most museum examples do not seem to have any swirls.
Could you give me an indication of the sorts of designs that these companies used for this type of glass? (website or book reference would help) and then I can pursue it further!
The early breakfast service by de Zwart and de Bazel was a strong vibrant yellow. Colopal (the breakfastware) was slightly paler 'primrose' shade, and in some (later?) examples became even lighter with a slight greenish tinge (nearly 'ivory').
The pudding mould 'leaf pattern' was produced in the stronger tint of yellow but the glass is actually only 'semi-opaque' if you look carefully.
The pudding mould 'classic dome' appears to be the same primrose shade as 'Colopal'