Bidens kept coming to mind because of the flower shape but I didn't think any of them were multi-colored. Every year I try and cut the weed back here before it goes to seed and every year I miss at least one of them and get covered in those double-tined seeds that just dig into your clothes. Most times I don't even know where I picked them up from. And it's never just a few, it's always hundreds of them that I have to pick off one by one...