Horses see blue as grey so if a showjump is on an arena surface that is grey it can blend in and make the get rails since they can't see it very well
It really depends on the horses but water jumps can be scary most horses are not really scared of the water it is the reflection of the water . But other bright colors spook horses
The horse is afraid of main colors or even of main shapes, because of their vision which is not like our own. They perceive colors less well than the human eye. To be more precise, the horse has a dichromate vision, ie a palette ranging from blue to yellow, short to medium wavelengths, but does not distinguish red or green (which appear gray to him).