All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Cutting nails is an act of Fitrah, so they should not be left for more than forty days without being cut. Anas said: "The Prophet prescribed for us a time limit for clipping the moustache, cutting the nails, plucking the hair under the armpits, shaving the pubes, that none of them should be left for more than forty nights."
Therefore, it is permissible for a woman to colour her nails with any colour provided the colour does not harm her and does not prevent water from reaching her skin when performing ablution.
Allaah Knows best.