A darker than normal staining pattern in the nucleus, known as Hyperchromasia
A clear area around the nucleus, known as a perinuclear halo.
Collectively, these types of changes are called a cytopathic effect; various types of cytopathic effect can be seen in many different cell types infected by many different types of virus.
When examining cytologic specimens, mild koilocytosis is characteristic of ASC-US (atypical squamous cells - undetermined significance). A more undifferentiated koilocyte, possessing a more hyperchromatic and enlarged nucleus, and a higher degree of cytoplasmic clearing with a discernible peripheral rim favor an interpretation of LSIL (low-grade squamous intraepithelial lesion).