What is a reasonable differential?
Usually, as in this case, there are few other possible diagnoses, although in general it is worth considering a calcified / hyperdense intraventricular meningioma, giant cell astrocytoma (usually with evidence of tuberous sclerosis elsewhere) or blood in the region of foramen of Monro.
What is the usual CT appearance of colloid cysts?
They tend to be unilocular and are typically hyperdense. Isodense or hypodense cysts are uncommon. Calcification is uncommon.
Non-contrast CT demonstrates a rounded hyperdense mass at the level of the foramen of Monro. The ventricles are enlarged, more-so than the sulci although there is certainly a degree of cerebral volume loss.