White aircraft would suggest that there are no liveries assigned to those aircraft in the FS Traffic fleet. You can check this by going to the Aircraft Manager page of the Traffic Control Centre and searching for that airline and livery combination. 

The easiest solution for removing white aircraft is to head to the Options page of the Traffic Control Centre app and set the Livery Substitution Mode to Alternative Livery. This will replace any white aircraft with an alternative livery. The Livery Substitution Mode can also be set to No Substitutions, so that only aircraft with correct liveries will be generated in the simulator.

You can also add and assign liveries to a specific aircraft type using the Aircraft Manager page in the Traffic Control Centre. Further details on how to do this are listed in the Aircraft Manager section of the manual.