Offspring colour

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How do you work out what colour foal you will get from a particular mating. I understand it depends on parents and grandparents but its all so complicated! Is there a simple way to work it out? I have a black mare who is by a piebald out of a dark bay, and a piebald mare of unknown parentage. Is there any way to tell what you would get if they had a foal?
Dad would either be black with black parents or piebald with unknown parents!:confused:

[Just.Jump]

http://www.vgl.ucdavis.edu/service/horse/coatcolortable.html

http://www.doubledilute.com/color-chart.htm#Colorpossibilities

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Thanks!