Acetic acid, ordinary table vinegar, is applied to the cervix. This causes the nucleus of the outermost cells to swell with water and not transmit light. The cervix is illuminated with light from the colposcope, the areas that do not transmit light reflect it back to the colposcopist. Reflected light is white, so white areas are sought. The outermost cells of the normal cervix have a very small nucleus and will transmit light, if they have a large round nucleus then they are dysplastic and will not transmit light.
White areas are the dysplastic areas.
The whiter the area the worse the dysplasia. Abnormal blood vessels can also be seen. Neoplasia means new growth. New growth means new blood vessels. Often these new blood vessels are abnormal.