I’d call the approach “dabbing on” each color. I’ve done variations on this, sometimes adding a hint of stone denim (another brighter blue) or even matte black instead of glossy black.

The only “rule” I’ve followed apply a light coat of the flux first — you don’t even need to cover the whole shell — then just keep dabbing on short brush strokes of each color *in any order* until every part of every shell is covered on the interior and exterior.