You can use anything. False flowers, to towels, fluorescent shirts, and like you said - paint. I would use any bright color that you can find! Red, blue green etc. If you Google show jumping pictures, you can get ideas about what colors are most commonly found, and what type of fillers are used.
Horses see colors differently that us, that is why jumps are commonly white and red. This helps the horse discern a jump from the ground. Keep in mind that once things are beyond about 3' in front of his nose, his vision becomes a bit like a cameral a little out of focus. Other than seeking the next nibble of grass, his eyesight detects movement but he can't necessarily make out what it is - thus he spooks or bolts.
Use anythings solid. We used to gather the christmas tress people threw out trim them down a little and put them under the fences, we also used corn stalks, build roll tops with wood from the hardware store, used coolers, sheets, etc.