So I was looking for Rainbow Thread to make various items. But the problem is: Where? The wiki said that it's available in Yuidora Mine but I can't find any nor any indication of it being there in the first place. And because Mercy Light Valley and Gushimera Temple is not available to me right now, Yuidora Mine is my only hope. Please help me with this as I'm nearing the chapter limit of some quest. :/

You can synth it from brown, blue, red and green strings.

And you need to place some furniture in Bedroom to increase your Knowledge (to >40 ,maybe).
It'll help add the recipe of Rainbow Thread to your Alchemy.