I used Ticks-Tocks' great tutorial on making custom pet coats, find it here.
For this upload to show up, you need to unlock the Neon Green fur for dogs; at Ticks-Tocks' site you will find how to do so. In the .rar is a Red XIII Sim I made; I'm not much of a pet sculptor but it will put the coat immediately in your custom pets folder. If you just want the coat, you can find it under Neon Green SMOOTH. Sometimes you have to look hard for it, and have to hit the golden retriever standard coat first. I recommend using my pre-made sim to find the coat.
Please bear in mind that this file will OVERRIDE the Neon Green Smooth coat for dogs; it's a DEFAULT replacement.