Hi and thanks for contacting Ask an Expert.
I don't think there is anything wrong with your beans. I think it may be a type of bean that has gray streaks. I looked up rattlesnake pole green beans and they look just like your picture.
Have you tasted the beans? Do they taste like regular green beans? Does the pod do anything else, such as getting soft or mushy? Or do they dry out prematurely?  
This is a very long link but it will show you the bean I looked at: https://www.bing.com/images/search?view=detailV2&ccid=UNK9S%2fxr&id=1CA5D9B5BB00D1564CC136E706F99CFC881C7CEC&thid=OIP.UNK9S_xr6Jxa2opi6Pl3rQHaE8&mediaurl=https%3a%2f%2fthereluctanthippie.files.wordpress.com%2f2015%2f02%2fimg_8603.jpg&exph=2848&expw=4272&q=Rattlesnake+Pole+Beans&simid=<personal data hidden>1994692&ck=F34C6CBE84C1A7EF97C9E8948E2D04E2&selectedindex=0&ajaxhist=0&first=1&scenario=ImageBasicHover  Just click on it to see the picture.
Is there anything else that you might feel is wrong with your green bean plants?  Actually there are a lot of different colored and streaked green beans.  Do you know the variety you planted?  If so, you might want to look it up and see what shows up on the Internet.
If you have further questions, please contact us again.