The horses will just be freaked out by the change, they are very wary of Thing That Are Different. For example I had a mare that was terrified of the daffodils every spring.
They are particularly wary of things on the ground that might get their feet. It makes sense for them to be suspicious of new footing and different coloured footing... might be a bottomless bog! Or a trap! Their vision is quite different from ours and they may perceive the markings as a potential hole in the ground even.
My daffy daffodil mare was also very cautious about normal road markings but was OK once they had proven themselves sufficiently safe and un-trapping. But she always high stepped over them just in case.
She'd have hit the roof at rainbow crossings! But would have eventually got used to it.
TL:DR... horses are Very Silly but it makes sense to them.