According to pomegranate expert Harvey Correia of www.purelypoms.com, cool nighttime temperatures aid in developing pigmentation, and sunburn on the fruit can lead to reduced color. We haven't had many nighttime temperatures below 85 degrees this summer, and several of the fruits did get fairly sunburned, especially the 'Garnet Sash' (aka 'Elf'). This seems very similar to blood oranges, which grow well in our area, but rarely attain the deep, dark color that the California ones do. In spite of the brutal weather, our pomegranate trees have been watered and well cared for, so it is quite possible that the fruits are just not developing internal pigmentation.
We cracked open one of our big red 'Unknown' variety (probably Wonderful) poms last weekend, and could see just a hint of coloration. The Elf/Garnet Sash was just as yellow as it has always been. We've been eating them all the same, and I expect to find hundreds of seedlings sprouting out of the lawn from where we've been spitting out the seeds. Boy #1 loves the pomegranates, although he sometimes gets carried away spitting the seeds, and winds up spitting out uneaten fruit.
The flavor of the Unknown/Wonderful variety is very good in my opinion, but I've been extremely disappointed with the Elf/Garnet Sash. Richard Ashton's pomegranate book described Elf as an ornamental variety, and I can believe it. It is a big yellow fruit that sunburns easily, and while it does get sweet inside, it is insipid and without any flavor. I planted it because of the description I read of it on the Urban Harvest website, and boy was I disappointed. I wrote Urban Harvest an email telling them about my experience, and suggested that they remove their incorrect description of Garnet Sash from their website, but as of this writing it is still at the top of their list. This winter I plan to either top-work the tree to another variety like 'Desertyni', or just chop it down and replace it with another variety like 'Eve'.