The text selection handle color in text widgets is primarySwatch[300] by default (https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/material/theme_data.dart#L135), but it should be the accent color.

The relevant guideline appears to be gone from the material documentation, but it's documented Google-internally at: https://material.googleplex.com/patterns/selection.html#selection-item-selection