Yellow Star charms are crafted in the Studio and are needed to restore the Yellow band of the Rainbow(0/30).
The items needed to craft them are as follows:
Yellow Star Charms +1 : 5 Beeswax, 1 Stone Blocks
Yellow Star Charms +3: 8 Beeswax, 2 Stone Blocks, 1 Alchemist Powder
Yellow Star Charms +6: 10 Beeswax, 3 Stone Blocks, 2 Starlight in Bottle