Powder is dropped inside the level of the dungeon with the corresponding name. For example to get Lavender powder you must kill monsters in the Lavender level of the dungeon.
As with most spawns in this game you are not going to get 1 powder drop per monster, it may take a lot of time …. having a Blue Control Crystal will help greatly as well.
In addition to this using a Pink Control Crystal to access the Recylatron or using Powder Boxes gained from Ancient Depth Chests will stand a good chance of getting some powder.