Im not sure if Im on the right track here but.... you dont have an HE of the spleen to look at.
For a starter - Id freeze in Isopentane if you're interested in morphology, forget slow freezing and use LqNit as a second resort. There are heaps of protocols but basically, use a disposable histo mold, fill with OCT, place in the tissue and freeze in your chilled isopentane, then store or cut sections.
Im not sure, but I suspect your white spots are lymphoid nodules in the spleen and these will appear paler than the surrounding splenic tissue. It would really help to see an HE.