So the winter witch archetype for witch can turn spells into cold as a swift action.
I believe this follows the rules for spellcasting modification that it applies to all casting you have.
So now say we get a spontaneous caster with the rime metamagic feat. Normally You can't rime a spell that you plan to change to the right descriptor, but with spontaneous I'm a little less sure since you don't need to prepare the rime version of the spell but apply rime on the fly.
Is the swift action able to turn it cold in time for rime to be able to be applied or does it already need to be rime-able and the swift action can't change that?
Both things happen when you are casting (changing it to cold and applying metamagic), so I am pretty sure you are able to choose the order. Obviously in this case you would choose to have the frozen caress apply first, making the spell eligible for rime.
Any other people care to chime in?
The rules are silent on order of operation. Both the swift action and metamagic feats are applied while casting the spell so I would allow it. However since there is no rule for official order of operations it will fall to the GM. If you are in PFS I would ask the GM's in the area of they will allow it.