According to the latest Nexus 6 rumor, the Sprint version of the phone will only come in the Midnight Blue color and will only come in the 32GB variant. It's entirely possible this rumor could turn out to be false, as it seems a rather odd choice. Of course, with Sprint we have seen surprising things before, so it will be worth waiting to see.
We can't imagine why they would make this choice, but it's possible Motorola is having a shortage in parts and/or offered some type of discount if Sprint would push the blue version exclusively. Of course, it's just as possible that this was all Sprint could afford at this time. We will give you a head's up if this intel is corroborated or not.
Here's our Nexus 6 section for further discussion: Nexus 6 Android Forum at DroidForums.net