Spotted Sandpipers do have two plumages, called in scientific language most commonly used basic and alternate plumages, also called breeding and non-breeding or winter plumage.  This photo is of an alternate or breeding plumb Spotted Sandpiper.  In basic, or non-breeding plumage, which it wears from after its molt post breeding season until spring just before breeding season it is grayer, lacks the spotted belly, and is quite drab compared to this bright alternate plumage. Hope this helps.  Good birding.  Steem on!