In "A Bird came down the Walk--" by Emily Dickinson, what does the phrase "too silver for a seam" mean? What do you think is suggested by the color silver?

This poem is a beautiful example of Dickinson's style, which often employs radically unique syntax and diction to craft images of astounding poetic quality. Unfortunately, because they are so tightly and uniquely crafted, Dickinson's images are often extremely difficult to interpret. The quote you've referenced is no exception to this rule. 
First, let's look at the quote in context:

... [the bird] unrolled his feathers, 
And rowed him softer Home -
 
Than Oars divide the Ocean,
Too silver for a seam,
Or Butterflies, off Banks of Noon, 
Leap, plashless as they swim. (15-20)

Based on the context the quote comes in, it seems like Dickinson is describing the fluid motion of the bird taking flight. Moreover, it seems like the phrase "Too silver for a seam" is meant to be a description for "the Ocean." The ocean that Dickinson has in mind appears to be seamless or smooth, as the absence of a seam is indicated by the phrase's syntax. Based on this assumption, we can assume that the phrase is meant to conjure images of a smooth ocean's surface, while "silver" is probably meant to evoke both a fluid but unbroken liquid form (such as might be found in mercury or "quicksilver," for instance), and to describe the color of the ocean's surface. As such, the phrase is basically comparing the motion of a bird taking flight to an ocean that is smooth and silver, although Dickinson characteristically complicates this interpretation with her unusual diction.  
 
As with all of Dickinson's poetry, this phrase is hard to interpret, and so my take on it is somewhat unstable and unsure. Taking my interpretation into account, apply it to the poem and see if it works, or whether or not your own opinion requires some more interpretive work.  
https://www.poetryfoundation.org/poems/56593/a-bird-came-down-the-walk-359