My four year old daughter has intermittent purple mottling of her skin over most of her body. she also has blue veins that are very noticable on her temple and along her jawline. She had a rigourous course of antibiotics at 5 weeks old for viral meningitis that I was told would be hard on her veins.
This is normal in this age no need to worry give her more fluid /water intake should be adequate as not Chat Doctor.