Use bright, flashy colours like neon. Use a lot of colours, too. You could use brightly-coloured poles if you have them, each pole a different colour. Then take a small plank of wood or something and paint it with flashy pictures and colours. Prop it up in the middle of the jump. Add flowerpots or windmills something under the jump. Decorate the sides of the jump with other things, like strands of tinsil.
If your horse is completely comfaterable with those, you've go a wonder of a horse. The horse I ride would flatly refuse!