The short answer is, "It varies." The long, drawn-out answer is that it varies, and I'll show you a few of them.
Let me start by saying this: The Rusty Crayfish is an invasive species and is one that has been horrendously detrimental to many of the fisheries in which it's been introduced. It's been confirmed in Iowa and Nebraska, and it's very, very easy to determine if it is, in fact, a Rusty crayfish by the spots on the side. If you see these guys, notify your local DNR office or Game and Parks representative.
In this example of a Rusty that was found in Omaha, NE in 2010. It's easy to see the green pumpkin, the brown, and the orange that is commonly associated with crayfish patterns.
Crayfish, generally, turn various shades of blue when they're molting. In many cases, they're a rather brilliant blue, however the colors tend to be rather subdued. Blue crayfish patterns are a relative staple for us in the Midwest, particularly in the spring of the year. (Crayfish patterns in general are exceptional in spring because of their availability.)
You can still see the spots on the side of this Rusty, if you look close. It's just that instead of being rust colored, it's now blue.
One of the best colors to use when crawfish are in this stage is the Blue Craw, or Sapphire Blue from Rage Tail. http://www.basspro.com/Strike-King-Rage-Tail-Craw-Softbait-Lures/product/96636/ Both colors are very good in IA and NE. At times, the Blue Craw is far superior because it has enough of the green In it that it shows a craw that is just beginning to molt- that's also when they happen to be the most vulnerable.
This is a Calico, or Paper Shell, that is also very common through the Midwest. Same blue colors that are apparent in most species of craws when molting.
In this picture, you can see where the highlights of orange and a primary dark green pumpkin are very prevalent. There are times, when fishing in clear water, you need to very closely match the colors of the craws. The way I do this is start with a base color like Green Pumpkin, and I dye the tips of the craw. I use a couple different methods to do this, but most of the time on the water, I carry a Spike It Marker with me that has Orange and Chartreuse, and red/Blue. http://www.basspro.com/SpikeIt-Scented-Double-Marker-Garlic/product/44444/ There are times, as well, that I will dye the tips of my craws before going out, then I place them back in the package, and use them as needed. When I'm going to dye a few at a time, I use a product called JJ's Magic for most colors, or I use Spike it in Colors that JJ's isn't available in.
I'm not ashamed to admit that I stole this picture from Steve Parks. He's the guy that invented the Rage Tail baits. and the guy that invented the Falcon lake Craw color. It also happens to be my most productive trailer, throughout the Midwest, and Texas. The pictures aren't superb, as to the color of Falcon Lake Craw, but when you see it up close, you understand why it's so versatile.
Falcon Lake Craw is essentially a Green Pumpkin base laminated to a rust-red. There's also some gold black and blue flake in it that really make a difference in the appearance of the bait in the water. you can fish it with a Green pumpkin jig to get more of the green to come out, or you can dye the tips more orange or chartreuse. You can add blue highlights to it as needed, as well. It's a standby in my trailer assortment, for absolute certain.
You can see from just these few pictures that the crayfish in our area come in a variety of colors. Far and away the best way to make sure that you've got your bases covered is to start with three or four primary colors- Green Pumpkin being #1, Falcon Lake Craw being #2, and a Blue Craw or Sapphire blue rounding it out.
Almost as important as color, is size. Having two or three different sizes of trailer is key in many situations. That alone can make the difference between catching fish, and just fishing.