>>534997156
Depends. White sparks on the parents and grandparents affect the chance to get the same spark on a horse, so what people do is make horses with lots of white sparks (especially ura finale) while not caring about the other sparks on it. They then use these horses to try and farm a horse that has both lots of good white sparks and the desired stat/aptitude.
For example this is a parent I'm using for spark farming, with triple ura finale and other good white sparks. Of course in the end, the chance to get the parent you want is still extremely small.
This is the lengthy process, but you can also just ignore white sparks and try to make parents with 9 blue stars.