What coat would this make?
 

My horse is pregnant by a big grey stud. So my horse is chestnut with a star and 1 white stocking, and the big grey stud, is a plain grey stud, what would possibly be the foals coat color?:D

Do you know what colour the stud was before he went gray?
Color Calculator If you do, use that website (click gray after putting his original colour).

It will most likely go gray however.

Depends on what base colour the stud is. What colour was he before he greyed out? Is it homozygous for grey or hetrozygous?
Your mare is ee for red but if the stallion is EE for black then you will get a black foal that is hetrozygous for black and red. If he's Ee you have a 50% of a chestnut and 50% of a black.
If the stallion is homozygous for the grey gene then he is GG and the foal will be Gg so will grey out. If he's Gg then you have a 50% chance of a grey foal.
Then there's the Agouti gene to play with.
Without knowing more it's quite hard to tell you really, there's a lot of possibilities depending on what the parents are carrying.

The easy way? 50% chance of grey. 50% chance of pretty much anything else =P

This, IF you don't know what his base color is, AND he's heterozygous for grey. If he's homozygous.. yer' gettin a grey babeh (well, eventually it'll be grey). :P