Their breeding stock is mixed color. Buff or orangish with black tails seem to be by far the most common color though. You don't have to select a rooster to match them in color.. unless you want to.
It can be hard to predict what colors yours will throw due to the mixed ancestry. Unless you have a black rooster, there will be no blacks as black is dominant. There is a recessive white so it is possible it may show up if they were carrying it. A flock of just one color is far more likely to be 'stable' for that color, yes. (especially if it's a color such as recessive white).
NN have a good reputation for being reliable egg producers(also by many comments here regarding them on this forum). There's a "but" though- in backyard flocks, it's extremely common for the hatchery stock to be mixed with various breeds with any naked necked chick being sold basically as a pure "Naked Neck/Turken".. I often see "Naked Necks" with tell tale signs such as small crests or leg feathering- signs they've been crossed/mixed.. there's the chance some of those may not be such good producers.
It's very commonly stated that any naked necks with any sort of neck patch are not pure however that is not true. There are many pure for the naked neck gene but the visual difference is these have a very small neck patch with just a few feathers on each side of the neck with the lower neck and crop area being naked and visible. Birds with large neck patch with the feathers covering the lower neck and crop area are not pure for the naked neck gene.
Birds with totally bare necks are not common.. they're around just don't see them that often.
Here's a rooster not pure for the naked neck gene(and note he has tell tale sign of a small crest)
This hen is homozygous. Note the lower neck is very visible and so is her crop area:
If you breed birds like the bird in second picture and none like the first, you will have a flock of birds pure breeding for the naked neck.