We used to have a hard time keeping our large, black dog cool. Last summer we bought a Ruffwear swamp cooler jacket and it has been incredible! You get it wet before you start hiking and as the water evaporates through the special fabric it really cools your dog down. Even when we are in the direct sunlight and it's really hot, Otis will be cool to the touch under the vest.  He loves it and it makes me way less worried about him overheating.  

If I'm hiking in an area without streams to rewet it, I will bring an extra liter of water with me to replenish the vest once it has dried off. It can be a little tricky to get it fully soaked by just pouring water on it (the water likes to just run off), so I bring a gallon Ziplock, and stick the vest in the Ziplock with some water and mash it around so it absorbs the water and I don't waste any!