Breeding help

I am looking to breed a Silver Grullo andalusian. I was wondering what genotype I needed to have or which ones I needed to look at to ensure a silver grullo or even just a grullo foal. 
I have bred a silver grullo colt before but I’m not seeming to get the same color from the same pairing as before. If anybody could help me it would be greatly appreciated. 

Grullo is dun on a black base, so you need the base color to be either EE aa or Ee aa (black coat with no agouti, which would turn it bay), and then need either DD or Dd (dun) as well. For silver, that's the "Z/z" gene, so you need it to either have ZZ or Zz to have silver.

I hope that helps!

Only The Wind hit the nail on the head.

It's a little easier to figure out what a foal could be using punnett squares. This thread explains genetics in depth: click here for guide

The reason you got a red dun is due to both parents being Ee aa - you had a 25% chance for an ee aa (chestnut), 25% for EE aa (black), and 50% for Ee aa (black carrying red). In addition to this, only the sire has the silver gene, but he has dominant ZZ. This means he passed it on and your filly carries silver as Zz, but since she's a chestnut based coat it won't show because silver only dilutes black pigment. The dun worked in the same way as the base coat, 25% for DD, 50% for Dd, and 25% for dd. Basically, you just got kind of unlucky with the odds this time.