That's probably normal and the color will change if you have time before the frost.

But the color won't change to the desirable orange, it will be a washed out white because it's not getting sun.

You can gently turn them so the orange side is down and that should get them to even up.

But if you don't have enough time before the frost there's not much hope.
If it's not normal you could have some kind of fungus or mold working and the pumpkin is rotting, in which case there's no hope at this point.

Ronnie:

There are many pumpkin varieties, with varying days to harvest. Larger pumpkins are usually later. What variety did you plant, or were the seeds saved from a store pumpkin?

Many pumpkin varieties can be grown well in northern climates, but they must be planted early. Where are you located, and when did you plant? You can speed the development of pumpkins, like most other vine crops, by planting through black plastic, which attracts the heat of the sun to the roots, and can help ward off common diseases like mildews on the vines. And, of course, pumpkins must be in full sun, also true of other vine crops. Are yours?

Don Yellman, Great Falls, VA

I live in Nashville. I don't remember exactly what kind I planted but they are the bigger orange ones, not white ones. I wasn't extremely interested in pumpkins but since this was my kiddos first experience at gardening, I thought they'd get a kick out of it. They are pretty big and bright orange on one side (ground-side), but still very deep green on the other. I turned them over today hoping that helps.

I had a really bad case of powdery mildew and lost most of my pumpkins to it and all the squash and zuchinni. But we had a really, really wet summer. (We've had 14 days of rain in a row.) But they are in a full sun spot. I'll probably skip the pumpkins next year and use that spot for other stuff.

I must say, with kiddos, it was an experience. We didn't get any watermelons cause they would pick them before they were ready, and it took a while before they learned that the cucumbers need to be bigger and tomatoes red. Funny story...I came home one day and they had ready for me about 20 cucumbers that were about as long as my thumb. The kids are 2 and 4 and really enjoying watch everything grow from seeds to stuff they see on their plates.

It's been a while since I decided I could make better use of my space growing something to eat other than pumpkin seeds and jack-o-lanterns (I can't stand pumpkin anything but the seeds), but I seem to remember it being the other way around.
The side away from the sun is the last to turn. They may be rotting.

I've also had very few bad years with pumpkins (this is billed as the "the pumpkin capital of the world"). I have had plenty of bad years with cantaloupes and if I remember correctly the side on the ground is most likely to go bad first when rotting from the inside (Alternaria?).

I had the best years growing medium sized jack-o-lantern types. When you consider the cost of grocery store jack-o-lanterns they were likely the most valuable thing in my garden. But then who in their right mind would buy that many jack-o-lanterns? My yard sure did look cool at Halloween.

If I had kids or even great nieces that lived close enough that didn't already have a huge garden of their own I'd still grow them.
If you grow corn you can grow the pumpkins in the corn patch without giving up much space. Give the early corn a good head start before planting the pumpkin seeds and stagger later and later corn varieties the further you get from where the pumpkins are planted.
If I have time to do that you should easily have time to do that. In fact even with my shorter growing season I had to learn to count backwards from Oct. so the pumpkins wouldn't get ripe too early.

I think your pumpkins are probably just fine, but need more time to turn color. It seems mine will turn first on the bottom, which I assume is because the bottoms don't get much light and don't have much chlorophyll, so they are basicly white. When they turn, the green chlorophyll fades and the other pigments show through and deepen, so without chlorophyll, the bottoms appear to turn first. Kind of the same process of leaves turning in the fall. Just a guess, though. At the point they are starting to turn, the pumpkin should be mature, just not orange.

Anyway, I often don't get my pumpkins turned to orange before frost kills the vines. If the skin is hard, I will bring them in and set them in a warm sunny spot in the house and they will usually finish coloring up in a couple weeks, which generally means I will have a traditional pumpkin at Hallowe'en. If not, I just put out green ones. Don't know if the sun is really necessary, but having them in a warm place as opposed to a cool, dark basement does seem to speed up the coloring process.