The Vallejo Sky Blue 961, 67 and the Testors 1508 seem to be a good base to start with. It is possible to make a mixture for the RLM78 if you can't find any proper one.
My reference says: RLM78 Azurblau - Humbrol 47, Gunze H418, Testors enamel 2087 what is the ModelMasterII RLM78 known as Hellblau.
It appears that the Model Master 4788 Hellblau RLM 78 Acryl is nothing more but the acrylic equivalent of the Testors/ModelMaster enamel 2087. So either the 2087 enamel or the 4788 acrylic paint you need.