Here are some screenshots (I hope they work):
These are the two horses I created in CAS. Please note the dark brown area is the part that is purple and the white areas are normally colored, while the lighter colored horse is slightly lavender. This will be important (I think).
Here is a golden colored horse I purchased. Note the mild pink tint.
Here is a white horse owned by a neighboring sim with no problem.
Here is a chestnut with similarly no problem, also owned by a neighbor.
My conclusions:
The purple coloring covers only hindquarters, back and neck areas of the horses.
The problem is only with created and purchased horses (I have not tested breeding or adopting yet).
The extent of the purple coloration is directly proportional to the darkness of the area affected. I suspect if I create a black horse, the purple will be very noticeable, while a white horse will be free of the defect (In fact, that will be my next test, along with some breeding - results will come in the future on that, if this is still a live topic).
goat