yup - most of the livebearers will eat their fry......you can get breeding nets which go in the top of the tank and allow the water to flow freely through the mesh. You can catch the babies carefully and put them into that and then they would be ok
The pigment discolouration - could be due to a number of things but you have to be extrememely careful what, if any, medications you put into a tank where there are young fry. IMO do frequent small (10% daily) water changes with water of the same temperature and pH - keep a close eye on mum platy for any changes in her skin, eating, swimming etc.
and do enjoy the babies.............they are just so cute
sue