Because you’re a bad cook? Nah, that can’t be it!
First of all, Rome or Rome Beauty apples are fine for making pies and crisps, which is good because they’re not great eating apples. So your choice of apple was not the problem.
There are two chemical reasons that your apples could have turned the color of dishwater. Most varieties of apples contain phenolic compounds and enzymes that discolor on contact with oxygen. When you slice the apples, you break through cell walls that have kept these substances separate and expose them to oxygen. Generally the color shifts to brown, but can also be gray. People put sliced apples in acidulated water - water with a bit of lemon juice, cream of tartar, or, best yet, vitamin C - to keep them from turning color before they are baked. Cortland and Golden Delicious apples have fewer of these phenolic gremlins and do not discolor as quickly as other varieties.
Heat also prevents discoloration, but we’re going to assume that your crisp went into a preheated oven, and that you cooked it at a reasonable temperature so that the apples didn’t change color while waiting and waiting for a little warmth.
Seems to us the most likely culprits are the tannins in apples. Tannins darken when exposed to even miniscule amounts of metal (as little as 5 parts per million). Where would your lovely apples have come in contact with metal? Yes, a wicked aluminum pan is one possibility. But it was probably tiny traces of metal present in hard water. If your water is even a little hard and contains metallic ions, in whatever quantity, you should consider using bottled water in your apple recipes.
You may have done everything right. It may be that you soaked your apple slices in a wonderful acidulated water to keep them from browning, and instead turned them gray because of the very same wonderful acidulated water. Isn’t that ironic?
Now, if you have color problems with other baked fruit desserts, don’t panic. Camouflage them with a dusting (light or heavy) of powdered sugar or cinnamon, top them with whipped cream, drizzle melted white chocolate liberally over each serving and place fresh mint leaves in strategic spots. Everyone who cooks occasionally turns out something ugly (even us), and knowing a few simple beauty secrets can be as useful as any fancy French cooking technique.
Finally, The Apple Cookbook, by Olwen Woodier, says Rome apples are indeed OK for pies and crisps, but Idared, Jonathan, Melrose, Mutsu, Newton Pippin and Northern Sky apples are "excellent" choices. So maybe in addition to taking steps to improve the color of your next crisp, you’ll be able to improve the flavor and texture, as well, by tracking down one of these varieties of apples.