In "The Vagabond," what does the line "white as the frosty field" mean?

The vagabond referred to in the title of this poem serves also as its speaker.  He is one who desires nothing more than to be a wanderer, living and sleeping outdoors, taking advantage of the freedoms allowed him by the road.  The second and fourth stanzas are the same, ending in these two lines:  “All I seek, the heaven above/And the road below me.”  Life on the road is all the speaker needs, in good times and in bad.  The line cited in your question falls in the third stanza, and is an image of those bad times.  The vagabond exclaims, “Not to autumn will I yield/Not to winter, even!”  By mentioning “White as meal, the frosty field,” the poet creates an image of winter, a field cold and frozen stiff, turned the color of porridge by the frost.  This image is used in conjunction with others, to the same effect – birds migrating away with the fall season, fingers turning blue with cold – and despite all these discomforts of the harsher season, the vagabond carries on with his wandering.  He refuses to be subdued, to fall prey to the warm comforts of a fire-lit room, but instead continues on in the elements, because it is in his very nature.  “There’s the life for a man like me,” he says at the end of the first verse, “There’s the life forever.”  And forever most certainly includes the freezing fields of winter.