Place the beans in the slow cooker, pour in enough water to cover them by four inches, and cook on LOW for eight hours.
Using an immersion blender, puree the red mung beans into a paste.
Transfer the pureed beans to a pot (it will still appear thin at this point), add the sugar and salt, and cook over medium-low heat, stirring often, until the mixture is a thick paste.
Cool the red bean filling.
Make the syrup
In a small pan, stir together the sugar and salt with a cup of water.
Drop in the pandan leaves and ginger slices.
Cook over high heat for five minutes.
Leave to cool (do not remove the pandan leaves and ginger).
Make the rice balls
In a bowl, stir just enough water into the glutinous rice flour to form a dough.
Divide the dough into three portions.
Add a few drops of red food color to one portion of the dough and knead until uniformly incorporated.
Add a few drops of yellow food color to another portion of the dough and knead until the coloring is even.
Divide each portion of the dough into one-inch balls.
Flatten each ball in the palm of your hand, place teaspoonful of read bean paste at the center and gather the edges to seal in the filling.
Bring a pot of water to a rolling boil.
Drop in the rice balls and cook until they float to the surface.
Serve the tang yuan
Scoop out the rice balls and divide among four bowls.