This Fine Liner Tool set from Europe is not available in stores or on the internet, here in the US.
The set contains two different tools. The one with the pointed tip is used to push the wet window color paint against the dried outline. It's very important that the wet paint touches the dry outline completely, otherwise it will separate when dry. You can also use it to achieve the effects in the butterflies (see video).
The one with the little ridge is used for blending and blurring colors.
The Fine Liner tools resemble brushes, but instead of bristles, they have a hard, plastic tip.
These are extremely helpful, and they don't wear out at all, even with heavy use. They clean very easily by either rinsing them under warm water,or just wiping them with a paper towel.